Showing papers in "AAPG Bulletin in 1983"

Genetic characterization of natural gases

Abstract: Natural gases can be characterized genetically using four properties: C2+ concentration, carbon and hydrogen isotope variations in methane, and carbon isotope variation in ethane. Three diagrams for genetic characterization of gases have been designed in which the carbon isotopic composition of methane is correlated with the other parameters. In these diagrams, compositional fields have been defined for primary gases (biogenic, thermogenic associated, and thermogenic nonassociated) and for gases which result from mixing of these gases. These fields are strictly empirical and comprise compositional variations found in about 500 natural gases. The isotopic and compositional variations in natural gases can be described in terms of (1) processes during formation of the gases such as bacterial fermentation or maturation of organic matter, and (2) processes during secondary migration. Mixing of primary gases is an important and common process. Migration of gases predominantly affects the C2+ concentration, whereas the isotopic properties of gaseous hydrocarbons primarily remain unchanged, allowing an assessment of the origin of migrated gases and properties of their source rocks. The formation of gas from humic organic matter and coals is not yet clear from published data. The diagrams use data from various basins and areas. Interstitial gases from the Gulf of California are entirely of bacterial origin: traces of thermogenic gases are formed only in the vicinity of dolerite sills; gases in the south German Molasse basin and in the Vienna basin are of bacterial, mixed, and thermogenic origins. Data from the north Italian Po basin provide examples for genetic characterization of migrated gases.

Correlation of Natural Gas by Use of Carbon Isotopic Distribution Between Hydrocarbon Components

Abstract: The natural distribution of carbon isotopes between hydrocarbon gas components is used for (1) determining a gas's maturity, (2) correlating a reservoired gas to its source, (3) correlating one reservoired gas with another, and (4) recognizing gas mixtures. Calculated separations of carbon isotopes between the normal alkane components of a natural gas have been related to source rock maturity by use of a single, continuous diagram, independent of source type. Actual data from a wide variety of geologic settings and geologic ages confirm this relationship and demonstrate its applicability to the source rock Levels of Organic Metamorphism ranging from 8 to 13, covering the entire range of oil and wet-gas generation. At greater maturities, the wet-gas components are found to undergo thermal degradation, losing their usefulness for correlation. Three examples showing indigenous gas (west Texas), non-indigenous gas (Gippsland basin, Australia), and gas mixtures from multiple sources (southeastern Alberta) illustrate exploration applications.

A Model for Development of Red Sea

Abstract: Although motion between Arabia and Africa is presently occurring along the entire length of the Red Sea, the morphology and tectonics that result from this motion vary greatly along its length. South of 21°N, the main trough is bisected by a deep axial trough which has formed by sea-floor spreading during the past 4 m.y. and is associated with large-amplitude magnetic anomalies and high heat flow. North of 25°N, an axial trough is not present and the floor of the main trough has an irregular faulted appearance. The magnetic field in the north is characterized by smooth low-amplitude anomalies with a few isolated higher amplitude magnetic anomalies commonly associated with gravity anomalies and in many places probably due to intrusions. Between these regions, the axial trough is discontinuous with a series of deeps characterized by large-amplitude magnetic anomalies alternating with shallower intertrough zones which lack magnetic anomalies. It is argued that the different regions represent successive phases in the rifting of a continent and the development of a continental margin. An initial period of diffuse extension by rotational faulting and dike injection over an area perhaps 100 km (60 mi) wide is followed by concentration of extension at a single axis and the initiation of sea-floor spreading. The main trough in the southern Red Sea, away from the deep axial trough, formed during the Miocene by the same processes of diffuse extension that are still active in the northern Red Sea. This model explains the available geologic and geophysical data and reconciles previous models for the formation of the Red Sea which emphasize either the evidence for considerable motion between Arabia and Africa or the evidence for down aulted continental crust beneath much of the Red Sea. The initial pre-sea-floor spreading stage results in considerable extension (160 km or 100 mi) at 25°N in the Red Sea), can last for several tens of millions of years, and is an important factor in the development of the continental margin. Such an extended phase of rifting and diffuse extension must be taken into account in studies of sedimentation, subsidence, and paleotemperatures.

Organic Carbon in Bakken Formation, United States Portion of Williston Basin

Abstract: The upper and lower members of the Mississippian and Devonian Bakken Formation in the United States portion of the Williston basin are black shales that are extremely rich in organic matter and are the source of much of the oil found in the basin. Organic-carbon values are calculated from formation-density logs using the equation: TOC = (154.497/^rgr) - 57.261, where TOC is organic-carbon content (wt. %) and ^rgr is formation density (g/cm3). Test calculations comparing this equation to laboratory organic-carbon analyses from 39 wells in North Dakota show an average absolute difference of 1.1% in organic-carbon content. Organic-carbon content, calculated at 159 locations in North Dakota and 107 in Montana, averages 12.1% for the upper member of the Bakken Formation and 11.5% for the lower member. There is a regional depletion of organic carbon, paralleling present-day isotherms, that reflects the conversion of organic matter to oil and subsequent expulsion of the oil from the formation. The mass of organic carbon in the Bakken Formation is approximately evenly divided between the upper and lower members, and it totals about 126 × 1012 kg in the study area, of which 102 × 1012 kg are in the thermally mature region. The assumption that 167 mg HC/g TOC have migrated out of the mature Bakken shales leads to a tentative estimate that hydrocarbons equivalent to 132 billion bbl of 43° (API gravity) oil have been expelled from the United States portion of the upper and lower members of the Bakken Formation.

Global Basin Classification System

Abstract: A proposed system classifies sedimentary basins, worldwide, into specific as well as general categories. The geologic history of each basin may be subdivided into cycles using three parameters: basin-forming tectonics, depositional sequences, and basin-modifying tectonics. Sedimentary basins may be simple, with one or two tectonic/sedimentary cycles, or they may be complex polyhistory basins with many different cycles and events. There are eight simple cycle types in this classification, which cover continental, continental-margin, and oceanic areas. The eight basic cycle types, their depositional fills, and tectonic modifiers have been given letter and number symbols so that the specific geologic history of each basin may be written as a formula. These formulas may then e compared and similarities or differences between basins noted.

Timing of Deformation in Overthrust Belt and Foreland of Idaho, Wyoming, and Utah

Abstract: A review of the timing and displacement evidence of the major structures of the western Wyoming Overthrust belt and foreland shows there is a progression in thrust displacement, apparent duration of motion, and palinspastic position of thrust traces from west to east. Those toward the west moved farther for an apparently longer period of time and are more widely spaced in their restored positions than those toward the east. However, average thrust velocities are all on the order of 0.5 ± 0.5 cm/yr (0.2 in./yr). Foreland events are in part synchronous with thrust belt events and had an effect on them. Although dating precision varies widely on major normal faults, present evidence does not contradict the generally held view that all normal faults postdate the youngest thrusting.

Petrofacies and Provenance of Late Mesozoic Forearc Basin, Northern and Central California

Abstract: Data from the Great Valley Group (sequence) represent the most complete information regarding sandstone petrology of sediment derived from a magmatic arc. This information is useful in documenting tectonic and magmatic events within the arc and related terranes, and forms the basis for the establishment of petrostratigraphic units for mapping and correlation. Sandstone and conglomerate compositions are controlled by changes in provenance, many of which were basinwide and synchronous. Clay-mineral composition is controlled primarily by burial metamorphism. Careful attention to sample collection, sample preparation, and petrographic techniques is essential for uniform results. Seven petrographic parameters (P/F, Lv/L, M, Qp/Q, Q, F, and L--listed in decreasing importance to petrofacies discrimination) define eight petrofacies (Stony Creek, Platina, Lodoga, Grabast, Boxer, Cortina, Los Gatos and Rumsey--listed in approximate order of decreasing age). The Upper Jurassic-Lower Cretaceous petrofacies (Stony Creek, Platina, and Lodoga) contain higher lithic contents (supracrustal sources), whereas the Upper Cretaceous petrofacies (especially the Rumsey) contain higher proportions of plutoniclastic components (quartz, feldspar, and micas). The proportion of potassium-feldspar increases from near zero in the Upper Jurassic to nearly 50% of all feldspars in the uppermost Cretaceous. The lower part of the Great Valley Group (Upper Jurassic and Lower Cretaceous) contains significant quantities of sedimentaclastic and metamorphiclastic material eroded from accreted and deformed terranes ("tectonic highlands") formed by the arc-arc collision (Nevadan orogeny) that occurred prior to initiation of the Franciscan-Great Valley-Sierra Nevada arc-trench system. The Klamath Mountains area provided a major proportion of this detritus. Ophiolite and serpentinite detritus was deposited locally near the base of the Great Valley Group as a result of deformation along the east side of the growing Franciscan subduction complex. Volcaniclastic detritus was fed into the entire forearc basin as magmatism increased in the Sierra Nevada area during the Cretaceous. As the volcanic cover was stripped off, plutoniclastic and metamorphiclastic detritus from the underlying batholithic terranes was provided in abundance to the forearc basin. Crustal components were more "continental" in the southern Sierra Nevada and more "oceanic" in the northern Sierra Nevada, as demonstrated by the higher proportions of metamorphiclastic detritus and by the more felsic nature of volcaniclastic detritus to the south. By the middle of the Late Cretaceous, extensive batholithic terranes provided potassium-feldspar-rich arkosic detritus to the entire forearc basin. By the Paleogene, arc magmatism had migrated eastward sufficiently that deeper levels of the California part of the arc were exposed by erosion, tectonic activity decreased in the forearc basin, and the basin was filled to sea leve in most parts.

Predictions of Oil or Gas Potential by Near-Surface Geochemistry

Abstract: Recent advances in surface geochemical prospecting have enabled age-old seep-detection technology to be used to determine the gas versus oil character of a potential fairway. Extensive field work has demonstrated that the chemical compositions of near-surface hydrocarbon soil gases, measured by flame ionization gas chromatography, are largely determined by the hydrocarbons in nearby underlying reservoirs. By using compositions and ratios of the light hydrocarbons, methane, ethane, propane, and butane, one may predict whether oil or gas is more likely to be discovered in the prospect area. Near-surface hydrocarbons are best represented by normalized histograms of composition data. These histograms are strongly correlative with those of reservoir gas and with compositions o gas from shows recorded in downhole mud logging. This correspondence with the actual formation gases suggests that the upward migration of reservoired light hydrocarbons into near-surface soils represents a viable mechanism, allowing surface geochemical exploration to be utilized for regional hydrocarbon evaluations. Geochemical profiles over known production areas are shown for the Sacramento and San Joaquin basins in California and for the Utah-Wyoming Overthrust belt. Geochemical predictions were documented by subsequent drilling near the Pineview field in Utah. The data imply that the Pineview field should extend westward into an area containing a dry hole. In addition, a new gas discovery--on the intersection of a Landsat lineament and a large methane anomaly--was made 6.5 km (4 mi) northeast of the Pineview field by Amoco in 1981. Most of the geochemical examples reported show direct anomalies over known fields. However, seeps can be laterally displaced in certain geologic settings. In addition, geochemical investigations indicate that seep magnitudes depend on tectonic activity to aid gas migration along faults and fractures, which appear to provide the major migration pathways. This fault association suggests that diffusion is of secondary importance. Geochemical prospecting must be used with caution, and only in conjunction with geologic and geophysical tools, because the location and shape of many geochemical anomalies are governed more by the local tectonic structure of the region than by the position and shape of the actual deposit. Regional groundwater flow is less significant. Thus, geochemical prospecting, when used alone, cannot predict whether a particular soil-gas anomaly is associated with a commercial deposit. It can only be used to verify the presence of petroleum hydrocarbons and to predict whether gas or oil is likely to occur in a potential structure. Geochemical prospecting yields excellent regional evaluations of hydrocarbon potential.

Eolian Dune, Interdune, Sand Sheet, and Siliciclastic Sabkha Sediments of an Offshore Prograding Sand Sea, Dhahran Area, Saudi Arabia

Abstract: An offshore prograding sand sea exists along portions of the Arabian Gulf coastline near Dhahran, Saudi Arabia. In this region, sediments of eolian dune, interdune, sand sheet, and siliciclastic sabkha intercalate with marine deposits. This depositional setting is characterized by strong offshore winds which supply abundant sand to the coastline, and cause at present time the outbuilding of the dune system. This quartz-detrital dominant setting contrasts markedly with the carbonate dominant setting resulting from onshore winds in the Trucial Coast area to the south. The broad intercalation of eolian and marine deposits which results creates ideal potential for subregional stratigraphic petroleum traps, due to pinch-out of porous and permeable dune sands into impermeable m rine mudstones. Within the eolian system itself are potential reservoir rocks (dunes), sources (organic-rich sabkha and interdune deposits), and seals (zones of early cementation in all deposits). Early cementation is very common in all facies of the eolian sand sea. The early cementation occurs owing to (1) soil formation, (2) deposition of pore-filling gypsiferous cements from saturated solutions near water table, and (3) addition of sand-size windblown evaporitic material to sands downwind of sabkhas.

Paleoaquifer and Deep Burial Related Cements Defined by Regional Cathodoluminescent Patterns, Middle Ordovician Carbonates, Virginia

Abstract: Middle Ordovician ramp carbonates, Virginia, were deposited in a subsiding, foreland basin bordered on the southeast by tectonic highlands. Ramp carbonates were lithified, in part, by turbid marine cements, but major cementation was by nonferroan, clear rim, and equant cements. Zoned (defined by cathodoluminescence) clear cements consist of nonluminescent (oldest), bright, and dull (youngest) cements; the zonation relates to increasingly reducing conditions of pore waters. Zoned cements in peritidal beds, best developed in southeastern belts, have complex zonations, pendant to pore-rimming fabrics, and are associated with crystal silt (which abuts all cement zones), solutional cavities, and erosional surfaces (which locally truncate dull cement). These cements are meteori vadose to shallow phreatic. Cements in northwestern exposures of peritidal beds are dominated by non-zoned, dull cement which lacks abundant evidence of early, near-surface precipitation. Major cementation of subtidal facies occurred under burial conditions. Burial cements, best developed in southeastern belts, have a simple zonation reflecting progressive burial (up to 7.5 km; 4.5 mi) of the carbonate ramp. Shallow burial nonluminescent cement formed from oxidizing, meteoric waters which expelled anoxic, connate marine waters. These meteoric waters were carried by aquifers from tectonic upland recharge areas on the southeastern basin margin. Bright cement formed under more reducing conditions following stagnation of the paleoaquifer with burial, and possibly was precipitated at depths of 2 to 3 km (1 to 2 mi). Burial cements in northwestern exposures of subtidal beds are dominated by dull cement, initial generations of which precipitated from downdip portions of aquif rs. Deeper burial, dull calcite and ferroan dolomite cements largely formed at burial depths of 2 to 3 km (1 to 2 mi) and temperatures of 75° to 135°C (165° to 275°F) or more, associated with hydrocarbon formation-emplacement during the late Devonian to Mississippian. Latest, clear dull cement fills tectonic fractures and was emplaced during late Paleozoic (Alleghenian) deformation, probably at temperatures of 200° to 300°C (390° to 575°F) and depths of 5 to 7 km (3 to 4.5 mi). Deeper burial diagenesis appears to be genetically linked to late Paleozoic, Mississippi Valley-type mineralization in the southern Appalachians. Zoned peritidal and burial cements are confined mainly to southeastern portions of the ramp where cementation was influenced by meteoric waters shed from tectonic uplands on the southeast and carried northwest by paleoaquifers. Northwestern portions of the ramp were influenced very little by upland-sourced, meteoric waters and nonzoned dull cements precipitated from relatively reducing waters. The distribution of nonluminescent cement in Middle Ordovician subtidal facies defines the regional distribution of oxidizing portions of the paleoaquifer system. Such incursion of upland-sourced, oxidizing meteoric waters into ramp carbonates should be a common feature of foreland basin carbonates deposited adjacent to tectonic uplands. Furthermore, foreland basin carbonates that undergo progressive burial should show a simple, nonluminescent-to-bright-to-dull sequence of cement zones. The close association of zoned cements and regional uplands in the Middle Ordovician sequence indicates the importance of assessing regional geologic relationships, environmental parameters, geologic history, and tectonics in understanding regional cementation patterns and cementation processes of anci nt carbonate platforms.

High-Porosity Cenozoic Carbonate Rocks of South Florida: Progressive Loss of Porosity with Depth

Abstract: Porosity measurements by borehole gravity meter in subsurface Cenozoic carbonates of south Florida reveal an extremely porous mass of limestone and dolomite which is transitional in total pore volume between typical porosity values for modern carbonate sediments and ancient carbonate rocks. A persistent decrease of porosity with depth, similar to that of chalks of the Gulf Coast, occurs in these rocks. We make no attempt to differentiate depositional or diagenetic facies which produce scatter in the porosity-depth relationship; the dominant data trends thus are functions of carbonate rocks in general rather than of particular carbonate facies. Carbonate strata with less than 20% porosity are absent from the rocks studied here. Aquifers and aquicludes cannot be distinguished on the basis of porosity. Although aquifers are characterized by great permeability and well-developed vuggy and even cavernous porosity in some intervals, they are not exceptionally porous when compared to other Tertiary carbonate rocks in south Florida. Permeability in these strata is governed more by the spacial distribution of pore space and matrix than by the total volume of porosity present. Dolomite is as porous as, or slightly less porous than, limestones in these rocks. This observation places limits on any model proposed for dolomitization and suggests that dolomitization does not take place by a simple ion-for-ion replacement of magnesium for calcium. Dolomitization may be selective for less porous limestone, or it may involve the incorporation of significant amounts of carbonate as well as magnesium into the rock. The great volume of pore space in these rocks serves to highlight the inefficiency of early diagenesis in reducing carbonate porosity and to emphasize the importance of later porosity reduction which occurs during the burial or late near-surface history of limestones and dolomites.

Source Rock Evaluation by Pyrolysis-Gas Chromatography

Abstract: A pyrolysis-gas chromatography system has been developed for the rapid evaluation of potential source rocks. For the determination of organic richness and maturation, this system uses the pyrolysis methods previously described. However, the determination of hydrocarbon production type and the identification of contamination (by migrated hydrocarbons or drilling additives) are accomplished by gas chromatographic (GC) analysis of the thermal extracts and pyrolysis products of rock samples or isolated kerogens. The production type is recognized either qualitatively by GC fingerprint traces or quantitatively by hydrocarbon composition (C1 to C6, C6 to C11, C11+) from the kerogen (Peak II) pyrolysate. Oil-prone erogens are recognized by GC traces with a full spectrum of C1 to C28 hydrocarbons, or by high concentrations of C11+ compounds. In contrast, gas-prone kerogens are characterized by the predominance of light hydrocarbons from C1 to C4 and higher contributions of aromatic compounds. Mixed-type production is intermediate in character between the two. Contaminants are identified from the GC analysis of the thermally extractable material in Peak I. Possible mineral-organic matter reactions during sample heating make interpreting data from whole-rock samples more difficult.

Reappraisal of Anoxia and Organic Richness, with Emphasis on Cretaceous of North Atlantic

Abstract: Accumulation of anoxic sediments in the North Atlantic during Barremian through Turonian time (Early to middle Cretaceous) was controlled mainly by local conditions, although global climatic and oceanographic factors played a supporting role. Evidence does not support the popular hypothesis developed over the past few years of global or oceanwide anoxia. Data which contradict this hypothesis are based on (1) a reevaluation of criteria for identifying sediments that accumulated in anoxic water, (2) a critical reappraisal of reported worldwide occurrences of anoxic sediments of these ages, and (3) an examination of the spatial and temporal distribution of anoxic sediments in the North Atlantic and elsewhere. A model is proposed in which the earliest anoxic sediments of Cretaceous age in the North Atlantic were deposited in deep, restricted basins. By middle Cenomanian time (about 97 m.y. ago), sluggish circulation had led to a gradual expansion of the oxygen-minimum layer, permitting deposition of anoxic sediment in nonbasinal settings as well. Expansion of the oxygen-minimum layer caused the calcite compensation depth to rise, promoting oxidation of organic carbon and causing contraction of the oxygen-minimum layer. Development of anoxia was thus self-damping. Anoxia effectively disappeared by early Senonian time (88 m.y. ago), when improved circulation created oceans more like those of the present. Some evidence indicates that enhanced biologic productivity and upwelling may have been local factors in fostering anoxia, but the Early and middle Cretaceous were generally not highly productive times.

Oil and gas prospecting beneath Precambrian of foreland thrust plates in Rocky Mountains

Abstract: Only 16 wells in the Rocky Mountain region have drilled through Precambrian rocks to test the 3 to 6 million acres of sedimentary rocks that are concealed and virtually unexplored beneath mountain-front thrusts. One recent test is a major gas discovery, another a development oil well, and over half of the unsuccessful tests had oil or gas shows. These wells have not only set up an exciting play, they have also helped define the structural geometry of the mountain-front thrusts, including the angle of the thrust, the amount of horizontal displacement, and the presence or absence of fault slivers containing overturned Mesozoic or Paleozoic rocks. Important for further geophysical exploration, these wells have provided vital data on seismic velocities in Precambrian rocks. A alysis of these data has stimulated further exploration along the fronts already drilled: in Wyoming, the Emigrant Trail thrust, the Washakie thrust, the Wind River thrust, the thrust at the north end of the Laramie Range, and the Casper arch; in Utah and Colorado, the Uncompahgre and Uinta uplifts. The geologic success of these wells has encouraged leasing and seismic acquisition on every other mountain-front thrust in the Rockies. An unsuccessful attempt to drill through the Arlington thrust of the Medicine Bow Range will probably only momentarily daunt that play, and the attempted penetration of the Axial arch in Colorado has not condemned that area; in fact, another well is being drilled at this time. Untested areas that will be explored in the near future are: in Wyoming, the south flank of the Owl Creek Range, the southwest flank of the Gros Ventre Range, the east and west flanks of the Big Horn Mountains, the west flank of the Big Horn basin, the north flank of the Hanna basin; in Utah, the south flank of the Uinta Mountains; and in Colorado, the White River uplift, the no th flank of North Park basin, and the Front Range.

Geochemistry and Habitat of Natural Gases in Po Basin, Northern Italy

Abstract: Three types of natural gases can be distinguished in the Po basin. (1) Bacterial gases (^dgr13CCH4: -60 ppt, ^dgrDCH4: -180 to -200 ppt, C2+: 0.2%), which are found predominantly in Pliocene-Pleistocene reservoirs. (2) Mixed gases (^dgr13CCH4: -60 to -50 ppt, ^dgrDCH4: -180 to -200 ppt, C2+: 0.2 to 5%, which are predominantly in reservoirs of Messinian (upper Miocene) age, but also in a few places in older and younger reservoirs. (3) Thermogenic gases (^dgr13C: -50 ppt, ^dgrD: -200 to -150 ppt, C2+: 5%), which commonly are associated with oil and are produced predominantly from Miocene and Mesozoic reservoirs. A quantitative assessment of gas in place for these three gas types is: (1) 80%, (2) 10%, and (3) 10%, respectively. The areal distribution and amount of the three gas types are controlled strictly by geologic factors. Synsedimentary tectonics and turbidite sedimentation are the best conditions for the trapping of biogenic gas in the southeastern part of the area, where 70% of the bacterial gas in place is produced. Here, bacterially formed methane is produced from depths of up to 4,500 m (15,000 ft), and is the deepest bacterial gas so far reported in the literature. In the pedealpine homocline, lower sedimentation rates and a weak deformation of Pliocene-Pleistocene sediments prevented appreciable accumulations; only 15% of the bacterial gas in the basin is produced in this area. Thermogenic gases are found in the vicinity of the Apennines, where strong tectonic movements have favored its migration from deeper Mesozoic sources into Tertiary reservoirs or into deep Mesozoic carbonate reservoirs of the pedealpine homocline.

Crustal structure of Ouachita Mountains, Arkansas; a model based on integration of COCORP reflection profiles and regional geophysical data

Abstract: COCORP deep seismic reflection profiles across the Ouachita Mountains in western Arkansas suggest that a large fraction of the crust in this region is composed of tectonically thickened Paleozoic sediments (and metasediments). Reflections representing a southward-thickening wedge of layered rock on the northern portions of the survey are associated with approximately 12 km (39,000 ft) of Carboniferous flysch overlying thin, lower to middle Paleozoic shelf strata in the Frontal thrust zone. Toward the interior of the mountain belt, the Benton uplift is a broad antiform, apparently cored by crystalline basement at depths below 7 km (23,000 ft). Beneath the southern Ouachitas and the adjacent Gulf coastal plain, a zone of south-dipping reflections probably represents at leas 14 km (46,000 ft) of tectonically thickened, lower to middle Paleozoic off-shelf strata and Carboniferous flysch. Regional Bouguer gravity data show a minimum coincident with the thickest accumulation of flysch in the Frontal thrust zone. To the south, the Benton uplift lies on a steep gravity gradient which is continuous along most of the Ouachita trend and which may be analogous to a gradient observed along the Appalachian chain. The Ouachita gravity signature can be modeled as a southward shallowing of the Moho (from 40 km [131,000 ft] in northern Arkansas to about 30 km [98,000 ft] just south of the Ouachitas), coincident with the tectonic thickening of the Paleozoic strata interpreted from the COCORP data. The resulting crustal section can be interpreted as the remnants of an early Paleozoic passive margin which was subducted beneath a thick accretionary wedge in Carboniferous time. The Bent n uplift is viewed as a late-stage involvement of crystalline basement in foreland thrusting as the margin entered the south-dipping subduction zone.

Evolution of Salt Structures, East Texas Diapir Province, Part 1: Sedimentary Record of Halokinesis

Abstract: Post-Aptian (post-112 Ma) strata in the East Texas basin were strongly influenced by halokinesis and therefore record the evolution of associated salt structures. Dome-induced changes in patterns of sandstone distribution, depositional facies, and reef growth indicate that thickness variations in strata surrounding domes were caused by syndepositional processes rather than by tectonic distortion. Salt domes in the East Texas basin exhibit three stages of growth: pillow, diapir, and post-diapir, each of which affected surrounding strata differently. Pillow growth caused broad uplift of strata over the crest of the pillows; the resulting topographic swell influenced depositional trends and was susceptible to erosion. Fluvial channel systems bypassed pillow crests and stacked vertically in primary peripheral sinks on the updip flanks of the pillows. Diapir growth was characterized by expanded sections of shelf and deltaic strata in secondary peripheral sinks around the diapirs. Lower Cretaceous reefs on topographic saddles between secondary peripheral sinks now host major oil production at Fairway field. Post-diapir crestal uplifts and peripheral subsidence affected smaller areas than did equivalent processes during pillow or diapir stages. Documented facies variations over and around domes at different stages of growth enable prediction of subtle facies-controlled hydrocarbon traps. Facies-controlled traps are likely to be the only undiscovered ones remaining in mature petroliferous basins such as the East Texas basin.

Fibrous calcite veins, overpressures, and primary oil migration

Abstract: The fibrous calcite veins, found parallel to the bedding of certain shales, may have been formed during burial as a consequence of overpressuring. Their presence could even be indicative of former overpressures. This suggestion, if it can be upheld, carries implications for theories of primary oil migration.

Chemical Constraints and Origins of Four Groups of Gulf Coast Reservoir Fluids

Abstract: Pitzer's method for computing activity coefficients has been applied to reported fluid compositions in four groups of Gulf Coast reservoirs: the Mississippi and Arkansas Smackover Formation, the Texas Edwards Group, The Texas Frio Formation, and the Louisiana Miocene sequence The activity quotients in fluids from carbonate reservoirs imply a reaction relation between calcite and disordered dolomite, in which ordering increases at higher temperatures This relationship indicates that an increasing Mg:Ca molality ratio with decreasing temperature does not necessarily imply dolomite dissolution by an updip-moving fluid Instead, it may reflect an approach to equilibrium between calcite and metastable dolomite Activity quotients in fluids from clastic reservoirs imply a rea tion relation between chlorite, illite, potassium feldspar, and quartz at temperatures above 100°C (212°F) Present fluid compositions in the Smackover and the Edwards appear to be related to the updip movement of Louann brines from the Gulf Coast basin, accompanied by dolomitization In particular, bromide plots for the Edwards fluids indicate simple dilution of Louann brines with interstitial fluids Similar plots for fluids from the Smackover show the complicating effects of illite formation, sylvite dissolution, and halite recrystallization Albitization may have occurred during fluid movement through clastics prior to entering the Smackover and Edwards Present fluid compositions in the Frio and Miocene are apparently not related to Louann brines The compositional data reflect the effects of halite dissolution, the subsequent albitization of plagioclase and potassium feldspar, and the dissolution of carbonates during secondary porosity formation

The Interaction of Natural Organic Matter with Grain Surfaces: Implications for Calcium Carbonate Precipitation

Abstract: Seawater is a complex solution of inorganic ions and organic molecules in contact with solid phases. Because of its reactivity and sorptive properties, some of the organic matter (OM) will directly affect the kinetics (and possibly the equilibria) of inorganic reactions by modifying the rates and types of reactions occurring between the inorganic ions and the solids. The interaction of natural OM with CaCO3 systems occurs in two ways: (1) adsorption of the OM to CaCO3 surfaces, and (2) complexation or chelation of free cations by dissolved or adsorbed OM. Both processes involve polar functional groups on the organic molecules, with the carboxylate anion (-COO-) being the most likely interacting species, although other functional groups may also be important. OM associated with a variety of skeletal and nonskeletal CaCO3, including skeletal organic matrix, OM within ooids, and OM extracted from carbonate grain surfaces, was studied for chemical characterization, adsorption phenomena, and cation-binding ability. Skeletal OM is largely protein, whereas ooid and adsorbed OM are humic substances with proteinaceous components comprising about one-third of the composition. Aspartic acid is the most abundant amino acid in both skeletal protein and the humic substances. Conversely, OM associated with noncarbonate End_Page 516------------------------------ sediments is poor in proteinaceous constituents and relatively depleted in aspartic acid. Aspartic acid-rich protein and humic substances bind or complex with metal ions in proportion to the concentration of carboxyl groups present. Blockage of carboxyl groups to make them inactive destroys the ability of the OM to bind metal ions. Many, if not most, of the carboxyl groups available for metal-ion complexation in both calcified protein and aspartic acid-rich humic substances are on aspartic acid. Thus, this amino acid provides a significant portion of the metal-binding ability of the different types of OM. Aspartic acid-rich OM is preferentially adsorbed by calcite compared to quartz. Again, the carboxyl group is the likely function to be involved in this adsorption. Blockage of carboxyl groups significantly reduces the ability of humic substances to adsorb to calcite. The similarity in geometry, charge, and composition enables the carboxylate anion (-COO-) to substitute for the carbonate anion (CO3=) in complexing calcium ion or adsorbing to calcite surfaces. Competition between organic and inorganic ions for dissolved species and surface adsorption sites is driven by the requirement of the system to remain electroneutral. Concentration variations in dissolved organic and inorganic ions in the pore waters brought about by bioturbation and organic and inorganic diagenesis result in variations in the tendency of OM to affect the chemistry of the system. Most of the CaCO3 formed in the marine environment consists of skeletal material. This CaCO3 contains proteinaceous OM that is thought to be involved in formation of the mineral phase (biological calcification). By analogy, naturally-occurring OM of somewhat similar composition and properties and with identical functional groups may also be involved in the precipitation f CaCO3 in the sedimentary environment (geological calcification). End_of_Article - Last_Page 517------------

Permo-Carboniferous Hydrocarbon Accumulations, Mid-Continent U.S.A.

Abstract: Approximately 19.4 billion bbl of oil and 119 tcf of nonassociated gas have been discovered in the Mid-Continent as of January 1, 1978. Although these volumes of hydrocarbons were trapped in thousands of fields throughout the Mid-Continent, the bulk of these resources were emplaced in a relatively few fields: about 14.2 billion bbl of oil have been found in 111 significant and giant oil fields, and 103 tcf of nonassociated gas have been discovered in 57 significant and giant gas fields. Permo-Carboniferous reservoirs are important in 101 of the large oil fields and 55 of the large gas fields; these fields contained 9.5 billion bbl of oil and 99 tcf of gas, respectively. Our calculations of the total oil and gas accumulations in Permo-Carboniferous reservoirs are extrapola ed from these data. About 2.1 billion bbl of oil and 5.1 tcf of nonassociated gas accumulated in Lower Carboniferous (Mississippian) reservoirs. Most of this oil and gas was stratigraphically trapped in Upper Mississippian sandstones and carbonates which are truncated at the pre-Pennsylvanian unconformity surface. Approximately 8.8 billion bbl of oil and about 31.9 tcf of nonassociated gas have been found in Upper Carboniferous (Pennsylvanian) reservoirs in the Mid-Continent. Most of these oil and gas accumulations were stratigraphically trapped in lenticular sandstone bodies; the environments in which the majority of these sandstones were deposited range from fluvial-deltaic to shallow marine. About 2 billion bbl of oil and 77.7 tcf of nonassociated gas accumulated in Permian reservoirs. Most of these hydrocarbons accumulated in rocks of Lower Permian age in the Hugoton-Panhandle-Panoma fields. The oil and gas in these fields were trapped by regional changes in lithofacies with associated hydrodynamic gradients. Significant stratigraphic accumulations of oil and gas will continue to be discovered in the Mid-Continent even though much of this region is now in a late mature stage of exploration and development.

Geocatalytic Reactions in Formation and Maturation of Petroleum

Abstract: Theory and experiments suggest that catalytic reactions are involved in petroleum formation. This suggestion is not new, but has never gained wide acceptance. A model chemical system illustrates that acid-catalyzed reactions can explain several key transformations in the generation and maturation of oil. It also is shown that such reactions should occur under geologically reasonable conditions, and that naturally occurring active catalysts exist in many sediments. Catalytic principles indicate that the nature and amount of these catalysts in sediments can importantly control the fate of organic substances therein. It is suggested that under a given time-temperature history, organic matter in a sediment may or may not generate petroleum depending on the concentration and a tivity of the geocatalysts. Stated another way, sediments differ in the time-temperature histories needed to generate petroleum.

Shelf and Deep-Sea Sedimentation in Eocene Forearc Basin, Western Oregon--Fan or Non-Fan?

Abstract: The lower Eocene Tyee and Flournoy Formations in the southern and central Coast Range of Oregon contain unusual associations of shelf and deep-sea facies deposited in an elongate portion of a forearc basin. The sandstone-rich nature and abrupt transition of sedimentary facies suggest that a variation of existing deep-sea fan models is present. The 6,500 ft (2,000 m) thick sequence calls for shelf sandstone virtually cascading into deep water along a line (shelf edge) rather than from a point source (submarine canyon) to feed a sand-rich system. Facies are described in terms of a sand-rich fan system. From south to north these facies include: deltaic, shelf, inner fan, mid-fan, outer fan, and basin plain. The deltaic facies contains coarse-grained, cross-bedded, distributary channel sandstone and interbedded coal. Some prograding deltaic sand extended to the edge of the narrow Eocene shelf and cascaded into the basin. Many complex, nested channels up to 1,200 ft (350 m) wide and 130 ft (40 m) deep were incised into fine-grained shelf and slope sediments at slope depths (line source). These channel and shelf-slope deposits comprise an inner fan facies. A single large canyon or feeder channel (point source) typical of some fan systems was not recognized. Mid-fan facies consist of broad, coalescing channels filled with thick, am lgamated sandstone beds. Sandstone/shale ratios in the mid-fan facies are from 85 to 15 up to 95 to 5. Outer fan facies have sandstone/shale ratios of 60 to 40 and contain well-developed, graded turbidite beds about 5 ft (1.5 m) or less in thickness, interbedded with shale. Basin-plain facies consist of fine-grained strata (sandstone/shale ratios of 30 to 70) deposited at the north end of the basin. These thin-bedded deposits also contain graded fine-grained turbidite sandstones. Paleogeographic reconstructions based on paleocurrent data, facies distribution, and sandstone composition indicate sediment derivation from the pre-Cenozoic Klamath Mountains terrane and a volcanic arc system to the south and southeast. The average sandstone composition is 19% quartz, 27% feldspar, 19% volcanic rock fragments, 7% other rock fragments, and 28% accessories and cement. Paleocurrent data from cross-bedding, and groove and flute marks indicate transport directions from south to north instead of a radially dispersed fan pattern. The stratigraphic succession of the Tyee-Flournoy system indicates a general shoaling trend and a progradation of deltaic facies across a narrow shelf and out over the basin fill during the early Eocene. The predominance of sandstone and confinemen within a narrow basin account for a turbidite facies differing from more familiar mixed sandstone and shale submarine-fan facies models.

Microfractures in Chalks of Albuskjell Field, Norwegian Sector, North Sea: Possible Origin and Distribution

Abstract: Production data from many North Sea chalk fields have indicated moderate to considerable contribution to production from natural fractures. This paper illustrates a detailed study of natural fracturing in the Albuskjell field, where gas/condensate hydrocarbons are contained in Upper Cretaceous (Maastrichtian) and lower Tertiary (Danian) chalks. The field is a large halokinetically induced dome located on the northern limits of the productive chalk region known as the Greater Ekofisk area. Examination of core material from wells 1/6-3, 2/4-F10, and 1/6A-10 (Albuskjell), has revealed the presence of two types of fractures. The first appear to be early, are commonly compacted, predominantly healed, and resemble conjugate shear fractures. The second type is related mainly to the tips of stylolites; these are vertical, preferentially open, and are interpreted as tension fractures. Tension fractures form, when the minimum effective stress is reduced to the tensile strength of the chalk, as a result of increased pore-fluid pressure and/or decreased total confining stress due to relative extension. Construction of effective stress versus progressive burial-depth profiles, using three models of overpressure generation, suggests that high pore-fluid pressures alone could not have formed the tension fractures in Albuskjell. Measurements and calculations from depth-converted seismic sections have shown that halokinesis occurred throughout chalk deposition and continued, in a series of pulses, until early Miocene time. Incremental stress values, associated with halokinesis, predict optimum conditions for tension fracture formation at 4,500-ft (1,370-m) burial, owing to a combination of halokinetic doming and overpressuring, with an important contribution from source rock maturation. These fractures possibly formed, therefore, post-hydrocarbon emplacement (~3,000-ft [1,000-m]) burial ^identity mid-Oligocene), and have been preserved by hydrocarbon invasion. The healed, shear fractures formed prior to significant pressure solution and hydrocarbon emplacement and are thought to be associated with extra-dome processes, possibly involving graben tectonics or halokinetic reactivation of earlier northwest-southeast-trending major faults. Their distribution is expected to be fairly uniform over the field, with broadly northwest-southeast orientations. Shear-fracture density is moderately constant between the Danian and Maastrichtian chalk sequences, but is significantly reduced, to absent, in the argillaceous base-Danian. Model studies predict that tension fractures should be concentrated in crestal areas of the field where near-random orientations may occur. Preferentially radial, with some concentric, orientations relative to the structural dome, may occur off the crest, with radial fractures persisting farther downflank. Predicted tension fracture orientations agree well with those measured from the oriented cores of well 1/6-A10. Open tension fractures are preferentially absent from tight (early-cemented or argillaceous) chalks. They are also poorly developed in "soft" lithologies and predominantly occur in chalks with moderate to high porosities with stylolites. The spacing of open tension fractures is assumed to be constant over the field, whereas fracture width decreases from crest to flank; this is in line with the concept of lower horizontal stresses in the structural crest. A plot of optimum fracture spacing versus fracture width has been constructed using strain values calculated from the halokinetic models. Analysis of production data supports the proposed model and agrees well with a strong halokinetic influence on fracture development. Although significant halokinesis ceased during the early Miocene, residual strain may still exist in Albuskjell, with larger horizontal effective stresses occurring in the flanks of the structure. Calculations suggest that tension fractures are probably open under initial field conditions to a potentia depth of 11,500 ft (3,500 m).

Geology and Hydrocarbon Accumulations, Columbus Basin, Offshore Trinidad

Abstract: The Columbus basin, situated on the eastern shelf of Trinidad, lies at the eastern extremity of a belt of severe deformation along the northern boundary of South America that has been affected by compressional and wrench tectonics in Pliocene-Pleistocene time. Two major structural trends are present in the Columbus basin: a series of east-northeast trending anticlines and north-northwest oriented normal faults. The basin was filled during late Miocene to Holocene time with sediments deposited by an ancestral Orinoco River draining a hinterland to the southwest. The Pliocene-Pleistocene section, which contains the hydrocarbon accumulations in the Columbus basin, was deposited in three coarsening-upward sedimentary sequences followed by a late Pleistocene transgressive sequ nce. Traps for hydrocarbon accumulation were formed by an easterly trending Pliocene-Pleistocene wrench system with associated east-northeast-oriented anticlines combined with north-northwest-oriented normal faults. Oil was sourced in the late Miocene lower Cruse Formation, whereas gas was derived both from Pliocene-Pleistocene pro-delta shales and as a late high-temperature phase of lower Cruse hydrocarbon generation. The north-northwest faults formed migration conduits from the oil source rock to Pliocene-Pleistocene reservoirs. The temporal relationship of faulting to oil generation is a major factor affecting the distribution of oil and gas. The size of hydrocarbon accumulations is limited to some extent by a lack of an effective hydrocarbon seal, particularly in the western half of th basin.

Evolution of a Forearc Basin, Luzon Central Valley, Philippines

Abstract: The Cenozoic history of the 14 km-thick Luzon Central Valley sequence illustrates the development of a forearc basin. Forearc basins are important both as major sediment traps and as sites of hydrocarbon accumulations. The Luzon basin is floored by oceanic crust on the seaward (western) side and older accreted terranes on the arc (eastern) side. Initial sedimentation on this oceanic crust occurred during early Tertiary northward translation and emplacement of the crust as an ophiolite along a strike-slip or oblique-slip zone. The basal sediments consist of pelagic limestones and thin ash layers overlain by sandy turbidites derived from uplift and progressive dissection of the ophiolite. A sequence of arc-derived sediments at least 26,000 ft (8 km) thick was shed into the astern (arc) side of the basin during late Paleogene to Quaternary convergence along the western margin of Luzon. By the middle Miocene, the Central Valley became a continuous, elongate basin fringed by extensive shelf deposits along both the uplifted seaward and arc sides of the basin. Detritus shed from both flanks filled the subsiding basin and resulted in progressively shallower depths. Nonmarine deposition began in central portions of the basin in the Pliocene and migrated with time both north and south along the basin axis. Late Miocene to Holocene movement along the Philippine fault zone caused uplift and folding of adjacent parts of the basin. Exploration models for the Central Valley predict gas-prone hydrocarbon generation in central portions of the basin at times that coincide with and postdate the formation of both structural and stratigraphic traps. Previous drilling in the basin has either been in areas with thermally immature source rocks or has failed to reach prospective intervals where thermal maturation is inferred. The hydrocarbon potential of the Central Valley has not been determined adequately.

Relation of Natural Gas Composition to Thermal Maturity and Source Rock Type in San Juan Basin, Northwestern New Mexico and Southwestern Colorado

Abstract: San Juan basin is a roughly circular, asymmetric structural depression located in northwestern New Mexico and southwestern Colorado. Ultimate recoverable reserves of predominantly nonassociated gas (23 tcf, 0.65 × 1012 m3) are present in the structurally low part of the central basin. The major producing intervals are low-permeability sandstone reservoirs in the Upper Cretaceous Dakota Sandstone, Mesaverde Group, and Pictured Cliffs Sandstone. Lesser amounts of oil and/or gas are produced from Pennsylvanian, Jurassic, and Cretaceous rocks along the southern and western flanks of the basin. The gases display a trend of becoming isotopically heavier (^dgr13C1 values range from -48.7 to -31.4^pmil and chemically drier (C1/C1-5 values range from 0.75 to 0.99) with increasing depth. These changes are assumed to be the result of thermal cracking processes, and the gases are interpreted to have been generated during the mature and post-mature stages of hydrocarbon generation. However, there is considerable scatter in the data which is interpreted to result from a difference in source rock type. Gases generated from nonmarine (humic) source rocks are isotopically heavier and chemically drier than those generated from marine (sapropelic) source rocks at equivalent levels of maturity. The gases also become isotopically heavier and chemically drier to the northeast, following the trend of increasing maturity of all units in that direction. The increase in maturity is attributed to a combination of greater burial depth and a higher geothermal gradient resulting from batholiths to the north in the San Juan Mountains area. Maximum burial and heat flow occurred during the Oligocene, which probably coincided with peak hydrocarbon generation. Lack of oil in the central basin is believed to be the result of two factors. First, gas in reservoirs such as the Dakota Sandstone may have resulted from thermal cracking of oil generated from marine source rocks during late mature (wet gas-condensate) and post-mature (dry gas) stages of hydrocarbon generation. Second, gas in reservoirs such as Mesaverde Group and Pictured Cliffs Sandstone is nonassociated and probably was generated from nonmarine (coaly) organic matter during the mature and post-mature stages. Minor amounts of condensate, instead of oil, may have been generated from nonmarine source rocks during the mature stage.

Evolution of Salt Structures, East Texas Diapir Province, Part 2: Patterns and Rates of Halokinesis

Abstract: The effects of salt mobilization on Aptian and younger (post-112 Ma) strata in the East Texas basin can be used to illustrate patterns of dome growth through time, and to estimate rates and amounts of salt movement. Pre-Aptian domes grew in three areas around the margin of the diapir province, apparently in pre-Aptian depocenters. Maximum dome growth along the basin axis coincided with maximum regional sedimentation there during the middle Cretaceous (Aptian, Albian, and Cenomanian). In the Late Cretaceous the sites of maximum diapirism migrated to the periphery of the diapir province. Diapirism began after pillows were erosionally breached, leading to salt extrusion and formation of peripheral sinks. The duration of pillow and diapir stages of growth was subequal, ranging from 10 to 30 Ma. Post-diapiric stage of growth continued for more than 112 Ma in some cases. Diapirs grew fastest in the Early Cretaceous, when peak growth rates ranged from 150 to 530 m/Ma (490 to 1,740 ft/Ma), declining in the Early Tertiary to 10 to 60 m/Ma (30 to 200 ft/Ma). Assuming steady-state conditions over periods of 1 to 17 Ma, strain rates for the rise of the East Texas diapirs averaged 6.7 × 10-16/sec; peak gross rate of growth averaged 2.3 × 10-15/sec, similar to slow orogenic rates. The evolution of East Texas salt domes essentially ended in the early Tertiary with uplift rates less than 30 m/Ma (100 ft/Ma).

Cretaceous Sinistral Strike Slip Along Nacimiento Fault in Coastal California

Abstract: The San Andreas and Nacimiento faults of coastal California both separate granitic and metamorphic basement rocks of the Salinian block from partly coeval but contrasting Mesozoic terranes underlain by the Franciscan subduction complex. By analogy with Neogene dextral strike slip along the San Andreas fault, Cretaceous sinistral strike slip can be inferred along the Nacimiento fault in preference to hypotheses for tectonic erosion during subduction or for dextral strike slip of unspecified amount. Following restoration of known San Andreas and inferred proto-San Andreas dextral displacements, reversal of about 560 km (350 mi) of postulated sinistral slip on the Nacimiento fault brings four major Mesozoic lithotectonic belts of California and Baja California into simple al gnment as subparallel terranes related to Mesozoic subduction along the continental margin. Neogene deformation within the San Andreas transform system involved (a) elongation of the Salinian block by dextral slip along subsidiary faults that branch from the San Andreas fault, and (b) dextral rotation of crustal panels within the Transverse Ranges. Latest Cretaceous and/or earliest Paleogene dextral slip along a proto-San Andreas fault followed the San Andreas course in central California, but diverged westward in southern California. Nacimiento sinistral displacements occurred in mid-Cretaceous to early or medial Late Cretaceous time, after Cretaceous emplacement of plutons now within the Salinian block but prior to deposition of uppermost Cretaceous sedimentary sequences in central California. Available data on Mesozoic relative and absolute plate motions in the Mesoameric n region support the likelihood of Cretaceous sinistral strike slip subparallel to the California continental margin. Paleotectonic reconstruction of crustal blocks in California and Baja California to their inferred mid-Cretaceous relative positions shows the Salinian block inserted on a bias between the flanking Mojave and Peninsular Ranges blocks. Salinian granitic rocks thus formed an interior part of the Mesozoic batholith belt, and their initial strontium isotopic ratios are compatible with the gradients displayed by values from the adjoining blocks. The similar Mesozoic terranes that now lie east and west of the Salinian block were then adjacent to one another west of the Sierra Nevada block. Available paleomagnetic data neither support nor preclude the reconstruction, but additional work together with future detailed lithotectonic comparisons potentially can confirm or refute the hypothesis i represents. A correct interpretation of the Nacimiento fault is important for understanding the overall tectonic framework of petroliferous basins both onshore and offshore in coastal California.

Clay Mineral Transformations in Tertiary and Mesozoic Sediments from North Sea

Abstract: Clay mineral transformations of 10 North Sea wells have been studied, and several transformation stages picked out and correlated with geothermal variations: smectite disappears at 149 to 167°F (65 to 75°C), whereas transformations in the mixed-layer illite-smectite clay minerals are observed between 113 and about 212°F (45 and about 100°C). At temperatures above 113 to 149°F (45 to 65°C), more than 30% illitic layers are found in the mixed-layered clay minerals, whereas temperatures from 140 to 167°F (60 to 75°C) indicate the transition zone toward more than 50% dehydrated lattice positions. Temperatures above 176 to 212°F (80 to 100°C) mark the lower limit for mixed-layer clay minerals containing more than 70% illitic la ers. The major part of the clay mineral transformations apparently takes place in the mesodiagenetic zone, in which most of the hydrocarbons are also formed.