Showing papers on "Petroleum reservoir published in 1972"

Effects of Depositional Environment and Postdepositional History on Chemical Composition of Lower Tuscaloosa Oils

Abstract: The crude oils in lower Tuscaloosa Cretaceous reservoirs in the central Gulf Coast fall into two groups on the basis of their chemical compositions. One of these groups appears indigenous to the lower Tuscaloosa interval. The oils in this group, all in unfaulted structural and stratigraphic traps, are in south-central and southwestern Mississippi, where the lower Tuscaloosa has been subjected to the deepest burial and greatest diagenetic influence. The second group of oils commonly is in lower Tuscaloosa reservoirs on faulted structures. The oils of this group are trapped where secondary migration routes across formational boundaries may have been created, or where younger and/or older source rocks have been brought into contact with the reservoir sandstones. The chemical compositions of the crude oils were determined primarily by chromatographic and mass-spectrometric methods. Most useful were the analyses of the individual light-hydrocarbon components, the sterane naphthenes, the saturate and aromatic hydrocarbon compound types, and the stable carbon-isotope ratios.

Geologic evolution and hydrocarbon habitat gippsland basin

Abstract: The Gippsland Basin of southeastern Australia is a post-orogenic, continental margin type of basin of Upper Cretaceous-Cainozoic age. Gippsland Basin evolution can be traced back to the establishment of the Strzelecki Basin, or ancestral Gippsland Basin, during the Jurassic. Gippsland Basin sedimentation commenced in the middle to late Cretaceous and is represented as a gross transgressive-regressive cycle consisting of the continental Latrobe Valley Group (Upper Cretaceous to Eocene or Miocene), the marine Seaspray Group (Oligocene to Pliocene or Recent), and finally the continental Sale Group (Pliocene to Recent). The hydrocarbons of the Gippsland Shelf petroleum province were generated within the Latrobe Valley Group and are trapped in porous fluvio-deltaic sandstones of the Latrobe. At Lakes Entrance, however, oil and gas are present in a marginal sandy facies of the Lakes Entrance Formation (Seaspray Group). The buried Strzelecki Basin has played a fundamental role in the development and distribution of the Cainozoic fold belt in the northern Gippsland Basin. The Gippsland Shelf hydrocarbon accumulations fall within this belt and are primarily structural traps. The apparent lack of structural accumulations onshore in Gippsland is largely due to a Plio-Pleistocene episode of cratonic uplift that was accompanied by basinward tilting of structures and meteoric water influx. The non-commercial Lakes Entrance field, located on the stable northern flank of the basin, is a stratigraphic trap and may serve as a guide for future exploration.

Rationale for Deliberate Pursuit of Stratigraphic, Unconformity, and Paleogeomorphic Traps

Abstract: Most basins contain facies changes, unconformities with resulting truncated beds, and buried erosional or constructive surfaces such as reefs, hills, channels, barrier sand bars, and other such phenomena--which form the basic requirements for the creation of subtle traps. If folding, normal faulting, thrusting, and the formation of salt ridges and domes are added to the picture of an evolving but continuously filling basin, the resultant structural and stratigraphic patterns become much more complex. However, no matter how complex the history, those stratigraphic relations and lithologic changes which are conducive to the formation of stratigraphic, unconformity, and paleogeomorphic traps remain. When hydrocarbon is expelled (primary migration) by pressure and heat from sediments which contain source material into adjacent reservoir rocks, it migrates through carrier beds (secondary migration) into sealed reservoirs, or traps. As long as the conditions necessary for secondary migration of a substantial amount of petroleum exist, migration will continue along strike and updip until all migrating hydrocarbons are either trapped in the subsurface or have escaped at the surface. As the petroleum moves, it will be captured by all traps--stratigraphic, unconformity, paleogeomorphic, structural, or a combination of these--which are in the path of migration. Because paleogeomorphic, unconformity, and stratigraphic traps are related (1) to older geologic surfaces, (2) to the location of strata on and directly below an unconformity surface, and (3) to lithologic changes within and laterally adjacent to a stratum, it is suggested that, in general, the conditions which produce most subtle traps are present before development of structural traps. If migration of hydrocarbons through a particular region were to take place before structural movements, all petroleum trapped during this early migration would be in subtle traps. Because subtle traps generally are formed as a result of constantly recurring depositional patterns which usually precede, or may be associated with, contemporaneous structural movement, petroleum basins probably contain more subtle traps than structural traps. Although much petroleum has migrated into structural traps, possibly more has accumulated in the earlier (and contemporaneously) formed subtle traps. Because subtle traps probably contain the large undiscovered domestic reserves needed for the future, explorationists must make the purposeful search for such traps an essential and substantial part of their exploration policy.

Geologic Outline and Oil Fields of Sergipe Basin, Brazil

Abstract: In the Sergipe basin of northeastern Brazil a Lower Cretaceous unconformity marks a change in general environment and tectonic style. Below this unconformity, Carboniferous to Lower Cretaceous beds are nonmarine, whereas the overlying Lower Cretaceous to Tertiary beds are dominantly marine. Intense normal faulting tectonic activity preceding the unconformity resulted in uplift and erosion which exposed Precambrian rocks in an area north of Aracaju while, in adjacent grabens, thick wedges of syntectonic conglomerates were deposited locally over older sediments. Irregularities on this unconformable surface were filled by the Carmopolis conglomerate and coarse sandstone member of the Muribeca Formation; the more extensive overlying Ibura Member evaporites of the same formati n also covered the areas where basement was still exposed. Neo-Cretaceous tectonism is characterized by small-scale faulting; the Riachuelo-Siririzinho and Vassouras-Carmopolis oil trends resulted from a combination of northwestward subsidence of basin-margin grabens and a regional southeastward tilting that started somewhat later. Oil production in this basin comes from the Carmopolis, Siririzinho, and Riachuelo fields, mostly from the Carmopolis Member. Some oil is produced also from Lower Cretaceous reservoirs in contact with the unconformity. Depth range of all reservoirs is 400-800 m. Favorable conditions for oil accumulation are the result of adequate structural evolution during Late Cretaceous time, presence of evaporites and probable oil source organic shales at the top of the reservoirs, and younger unconformities not reaching down to the trap. Locally, lateral permeability barriers or reservoir pinchouts complement the structural trap closure. The oil is of mixed base ranging from 24 to 30.5° API. Cumulative production to December 31, 1969, with Siririzinho and Riachuelo fields still being developed, was 23.65 × 106 bbl of oil. This production comes from 205 completed wells drilled in a 400-m grid.

Treatment of gravel packed formations

Abstract: The treatment of formations, which have been previously packed with gravel, with, for example, an acidizing liquid can be improved by employing as a diverting agent in said treatment fluid particulate diverting agents having a size distribution such that at least about 75 percent of said diverting agent passes through the pack.

Reservoir evaluation and deliverability study, Bierwang Field, West Germany

Abstract: This report summarizes the results of the numerical model studies performed on the Bierwang field gas reservoir during April-June 1971. The primary objective of the study was to predict the future deliverability of the reservoir under the influence of the associated aquifer and to determine the expected ultimate recovery. The original gas in place was determined volumetrically as well as dynamically with a linear programing model. This gas in place can be supported with a history match of pool performance utilizing a realistic reservoir description. The ultimate gas recovery of the pool could approach 74% of the original gas in place under the proposed production schedule averaging 75 MMcfd. The expected ultimate gas recovery would only approach 69% of the original gas in place, if the gas production were maintained at an average 50 MMcfd. Pool deliverability will be sufficient to meet the maximum gas take to the middle of 1973 under the proposed drilling program and production schedule. The aquifer is extremely active with a large degree of nearly pit piston-like water entering the gas zone. Observed water production is believed due to a combination of aquifer influx and water channelling in isolated high permeability streaks. (12 refs.)

Permeability improvement technique

Abstract: A water sensitive subterranean fluid reservoir, a petroleum reservoir, for example, previously damaged by swelling due to contact with water is improved in permeability by treating the reservoir with potassium chloride at about 100* C. for several hours.

Stratigraphic-Trap Classification: Geologic Exploration Methods

Abstract: A trap for hydrocarbons requires the simultaneous existence of (a) a reservoir, (b) an isolated region of low potential in the reservoir, and (c) a barrier (or seal) with high enough entry pressure to retain a commercially producible volume of hydrocarbons. Three kinds of traps exist--structural, stratigraphic, and hydro-dynamic. All three kinds have a reservoir bounded by a barrier but differ in what causes the isolated area of low potential. In classification of hydrocarbon accumulations, the conditions that determined the present location of the accumulation should be used where they can be ascertained. In the stratigraphic-trap classification suggested here, primary emphasis has been placed on usability--i.e., will the groupings help in the search for new hydrocarbon accumulations, and is the suggested terminology simple and descriptive enough to be accepted? A classification using the time relations between barrier and reservoir was considered and rejected. The suggested classification starts with the simple concept that stratigraphic traps are adjacent to unconformities or they are not. For traps that are not adjacent to unconformities, the reservoir and barrier may be (I) primary (depositional, usually facies-related) or (II) wholly or in part secondary (diagenetic). Those traps in contact with unconformities may be (III) below the unconformity surface or (IV) above it, or (V) both below and above it. This approach uses some of Levorsen's ideas and eliminates some inconsistencies in his classification. Subdivision of these four major classes (facies-change traps, diagenetic traps, traps below unconformities, and traps above unconformities) allows more precise description of the different types of traps.

Sergnano gas field, Po Basin, Italy--a typical stratigraphic trap

Abstract: Gas accumulation in stratigraphic traps discovered to date in the Po Basin is limited to the Pede-Alpine region. The Sergnano field is a clear example of a stratigraphic trap. The gas reservoir is a sand-gravel body, closed by abrupt facies variation into lowermost Pliocene shale; a transgressive basal conglomerate of Pliocene age overlies truncated lower Miocene-Oligocene strata. Reflection seismic surveys were the determining factor in the discovery of this kind of trap. However, a contribution to the detection of similar but less evident stratigraphic traps was made by a device developed in the research department of AGIP Mineraria.

Stratigraphic Trap Potential, Permian Phosphoria Formation of Central Wyoming: ABSTRACT

Abstract: There are good opportunities for finding stratigraphic traps in the Permian Phosphoria Formation of central Wyoming. Ideally, hydrocarbons may be trapped in an updip reentrant of porous carbonate rock extending into redbed-anhydrite facies or in a porous algal leaf and oocastic mound enclosed in impermeable carbonate rock. Three facies are found as bands of dolomitized carbonate rocks on the Wyoming shelf between the marine limestones of the Phosphoria Formation and redbeds of the Goose Egg Formation. Westward, the shelf deposits grade into basinal phosphatic and cherty beds. Two transgressive-regressive marine cycles are present in which limestone grades through dolomite into a redbed-anhydrite facies. Stratigraphic work on the carbonate members (Ervay, Franson, and Grandeur) has defined three mappable facies exhibiting porosity and permeability due to secondary dolomitization. These facies consist of (1) algal oolitic-pellet mounds, (2) algal pellet mounds, and (3) oocastic carbonate rocks. Traps formed where the algal oolitic-pellet mounds of the Ervay Member grade into impermeable redbed-anhydrite and dense carbonate rocks have proved the most important economically. Cottonwood Creek field, containing 45 million bbl of reserves, is an excellent example. This facies relation can be traced from outcrops in the northwestern Big Horn Mountains, through the Big Horn basin, to wells drilled on the south flank of the Wind River basin, where drilling so far has failed to find a productive trap. The Laramide orogeny further complicated the conditions of entrapment. The oocastic facies, best developed in the Ervay and Franson Members, is directly west of the algal oolitic-pelletal facies and trends as a band across central Wyoming. This facies has excellent porosity but poor permeability, and has produced mainly from fractured reservoir rock on structures such as Winkleman dome, Circle ridge, and Beaver Creek. The algal-leaf facies, found as lenses in fine-grained nonporous carbonate rock, parallels the depositional trends of the other facies; it also is interbedded with the algal oolitic and oocastic facies of the Ervay and Franson Members. The algal-leaf facies has excellent capacity and, if oil-saturated, constitutes an excellent reservoir. This facies is the primary pay zone in No Water Creek field in the Big Horn basin. Identification of this facies is difficult in wells, so cores and thin sections are needed. End_of_Article - Last_Page 601------------

Star-Lacey field, Blaine and Kingfisher Counties, Oklahoma

Abstract: The Star-Lacey field, on the N. shelf of the Anadarko Basin, produces from stratigraphic traps in the Hunton Group, as well as from Mississippian limestone. The dolomite zones in the Hunton are the important producing zones. Because the development of secondary dolomite porosity is known to be related to erosional surfaces, recognition of unconformities is important in exploration. Porous dolomite zones in the lower Hunton that are not associated with pre-Woodford erosion may be related to structural topographic highs. A line of structural noses is apparent in the Star-Lacey area. Estimated total ultimate reserves of the Star field are 6,750,000 bbl of oil and 30 billion cu ft of gas. Southwest Lacey field has estimated recoverable reserves of 5 million bbl of oil plus a substantial amount of gas.

West Campbell (Northeast Cedardale) Gas Field, Major County, Oklahoma

Abstract: The West Campbell field is a prime example of the principal type of Hunton stratigraphic trap in the Anadarko Basin. Gas and condensate are produced from a lens of dolomite within the predominantly dense limestone of the Hunton Group of Silurian-Devonian age. The field was discovered by drilling on a seismic anomaly with unproved north closure in an area where subsurface control indicated the presence of porous and permeable Hunton dolomite. Later drilling proved that the trap was stratigraphic. Because the Hunton hydrocarbons are trapped almost entirely within the dolomite facies, it is essential in exploration for further traps of this type to relate the origin of the dolomite members to the history of deposition, diagenesis, and oscillations of sea level in a developed field such as West Campbell. As evidenced by various minor unconformities in localized areas, the sea oscillated frequently during the time of Hunton deposition. It is believed that, at each period of prolonged stability of the sea, dolomite was deposited all along the shoreline in shallow water. As a result, several trends of dolomite parallel the old Hunton shoreline.

Star-Lacey Field, Blaine and Kingfisher Counties, Oklahoma: Case Histories

Abstract: The Star-Lacey field, on the northern shelf of the Anadarko basin, produces from stratigraphic traps in the Hunton Group, as well as from Mississippian limestone. The dolomite zones in the Hunton are the important producing zones. Because the development of secondary dolomite porosity is known to be related to erosional surfaces, recognition of unconformities is important in exploration. Porous dolomite zones in the lower Hunton that are not associated with pre-Woodford erosion may be related to structural topographic highs. A line of structural noses is apparent in the Star-Lacey area. Estimated total ultimate reserves of the Star field are 6,750,000 bbl of oil and 30 billion cu ft of gas. Southwest Lacey field has estimated recoverable reserves of 5 million bbl of oil plus a substantial amount of gas.

Oil Column Calculations in Stratigraphic Traps

Abstract: Oil columns can be calculated for simple stratigraphic traps if the rock and fluid properties are known or can be estimated. Because oil migration is prevented by capillary pressure in small pores of the trap facies, direct measurements of capillary pressure allow oil columns to be calculated, but such measurements are rare. An alternative is to determine pore size from porosity and permeability data using an empirical equation (Berg, 1970), and then to compute the capillary pressure by an estimate of fluid properties. An example of oil column calculation is the Milbur field, Burleson County, Texas, a lower Wilcox stratigraphic trap. Using core analysis for a nearby well, an oil column of 40 to 70 feet would be expected for the trap, and this estimate agrees reasonably well with actual oil columns of 60 to 75 feet for the field (Chuber, 1972). The most important part of such calculations is the realization that the trapping facies itself can have significant porosity and permeability and yet form an effective barrier to oil migration. The result is that the best oil reservoir may occur down dip from dry holes with porous water sand and oil shows, rather than up dip at the pinchout.

Exploration Methods for Stratigraphic Traps in Carbonate Rocks: Geologic Exploration Methods

Abstract: It is commonly impossible to distinguish structure due to deposition and erosion from tectonic structure. Moreover, stratigraphic agents producing trap limits are usually dependent on tectonic influences. It is thus impractical to contrast stratigraphic versus structural traps with the intention of searching for one type and not searching for the other. This is most clear in the case of carbonate traps where tectonism controls development of both erosional and depositional structure and affects depositional and diagenetic facies distribution. In the Florida-Bahamas carbonate province, intra-platform straits and basins are sites of negative residual Bouguer gravity anomalies. A correlatable refractor near the top of Lower Cretaceous rocks is depressed in these same areas. Thus, present topographic lows overlie structural lows in the platform's foundation. Similar relations are indicated for the Tampico-Tuxpan and Scurry reef platforms and are markedly evident in the Central Basin platform and the Leduc-Rimbey trend. This relation is a potentially useful one, because the geophysical anomalies reflecting the structures which control the position of the platforms commonly exceed those stemming directly from carbonate masses. Depositional and, to a degree, diagenetic facies have consistent topographic settings in both recent and ancient platforms. Calcarenites predominate at the edge; calcilutites and evaporites are most common in the platform interiors. Bases of platform-edge slopes are typically sites of deposition of allochthonous shallow-water sands mixed with coarse rubble containing balls of pelagic mud. Elevated edges commonly are leached and dolomitized, and dolomite is present in many places within the platform-interior evaporites. Great quantities of hydrocarbons have been found in the leached and dolomitized platform edges, in porous and permeable platform-interior dolomites, in dolomitized conglomerates bordering bases of platform slopes, and in fractured reservoir rocks in adjacent basin facie . The requirements for an oil field are structure, reservoir, seal, and a commercial quantity of hydrocarbons. Geophysical tools are best suited to discern structure. Velocities and reflection character also provide some insight to lithologic variations. Outcrop and subsurface studies enable mapping of distribution of reservoirs and seals. Slabbed cores from ancient carbonate rocks reveal sedimentary structures identical with those observed in recent carbonate units. Thus, study of modern carbonate deposits is a valuable aid in interpretation of rocks, and slabbed cores are essential for a detailed understanding of carbonate depositional and diagenetic history. Temperature and hydrocarbon-generating history of source beds can be discerned from the nature of organic matter remaining afte oil and gas are gone. Knowledge of this relation enhances the ability to predict types of hydrocarbons to be encountered in a given region. Prospects for testing must be chosen on the basis of areal extent of structure and the regional distributions of reservoirs, seals, and hydrocarbons. Finally, management and backers should be prepared to drill two or three evaluation wells following the completion of a successful wildcat in a carbonate reservoir.

Oil-Water Transition Zone in a Permeability Trap, with Reference to Free Water Surface

Abstract: This paper deals mainly with the distribution of oil and water in a reservoir with gradual change in rock charateristics, from a viewpoint of reservoir geology. In this paper, water-wet sandy reservoirs under hydrostatic condition are considered. (Reservoirs under hydrodynamic condition will be discussed in the near future.)Before taking up the main subject, it is necessary to make the concept of free water surface clear. In a sand reservoir of uniform rock characteristics, a cave of supercapillary size is assumed. An oil-water interface in the cave is the free water surface (Fig. 1A). If oil and water have a phase continuity in the reservoir respectively, and if the displacement pressure of the reservoir sand is not zero, relationships between water saturation and height, and between fluid pressures and height, are shown in Fig. 1B and 1C respectively. However, the shape of capillary pressure curve changes with the change of reservoir rock characteristics, as shown in Fig. 2. Fig. 3 shows a generalized relationship between relative permeability curves and the oil-water transition zone. This figure is drawn taking into consideration the free water surface and displacement pressure.In case where there is a lateral and gradual change in reservoir rock characteristics, Fig. 4 was proposed by J. J. Arps (1964). I have revised this figure taking displacement pressure into consideration (Fig. 5). In any event, as a thick reservoir rock changes laterally and gradually from well-sorted coarse-grained clean sand to poorly-sorted fine-grained shaly sandstone, oilwater transition zone will certainly get thicker and be shifted upwards above the free water surface.Based upon these considerations I discuss the distributions of oil and water in a permeability trap. Fig. 6 shows a gently dipping reservoir, in which well-sorted coase-grained clean sand changes gradually into mudstone in the updip direction, via poorly-sorted fine-grained shaly sandstone. When oil is trapped in this reservoir, a sharp oil-water contact is formed on the downdip side. Above this contact oil reservoir with a small amount of interstitial water exists. Further up the dip, water saturation is considered to be getting larger, forming a wide oil-water transition zone. Provided that the first exploratory well (Well 1) was encountered with the transition zone, the second well (Well 2) will be drilled on the updip side, so long as the geometry of the sandstone body and its lithologic characteristics are unknown. But, Well 2 will be a dry hole. Well 3, located on the downdip side, will produce oil.I think such a type of oil pool as shown in Fig. 6 is likely to be often overlooked, and many such pools may be awaiting discovery. How to find out this type of pools by the minimum number of wells is an important practical problem. Sedimentological analysis of cores, high density analysis of continuous dipmeter logs, shape analysis of S.P, curves, etc. in connection with the sedimentological and geohistorical studies on the surrounding area seem to be most effective.

Flow in fissured rocks and effective stresses. Application to hydrocarbon production and to hydraulic fracturing

Abstract: Production of fluids from fissured reservoirs generally decreases with the increase in effective stresses being exerted on the formation. Inversely, reservoir rock permeability in the vicinity of a well increases progressively during hydraulic fracturing as the result of the opening up of natural fissures followed by the development of a main fracture running perpendicular to the smallest main stresses. Variations in rock permeability as a function of variations in effective stresses occur in a reservoir during production and hydraulic fracturing and may be plotted as a single curve whose shape depends on the rate of rock fissuring.

Petroleum and geology: a synthesis

Abstract: A marine sedimentary basin typically begins with a transgressive phase and ends with a regressive phase; but there may be several cycles, and also periods in which neither is dominant. Petroleum occurrences fall into two broad stratigraphie classes: those of transgressive sequences, and those of regressive sequences. Transgressions tend to accumulate potential source rocks on top of potential reservoir rocks, and the petroleum tends to migrate downwards then laterally into stratigraphic traps, especially reefs and below unconformities. It also occurs in diachronous units that are anticlinal in form due to basement irregularities. Regressions tend to accumulate potential reservoir rocks on top of potential source rocks, and the petroleum tends to migrate upwards and then laterally into anticlines and fault traps that are typically initiated contemporaneously or penecontemporaneously with sediment accumulation. There is some evidence that oil of transgressive sequences is heavier than oil of regressive sequences. Evidence derived from subsurface geology, including petroleum occurrences, suggests that young marine sedimentary basins are typically deformed by vertical, gravity processes during and just after significant regressive phases of their development; and that these processes are a direct consequence of the accumulation of sediment in a regressive sequence. Subsequent horizontal tectonic events, in general, only modify the earlier, contemporaneous, deformation.

Candeias field: typical stratigraphic traps

Abstract: The Candeias field, in the S.-central part of the Reconcavo Basin, Brazil, was discovered by Conselho Nacional do Petroleo (CNP) in 1941 as a result of surface geology and seismic interpretations. The reservoir rock consists of fractured lenticular sandstone (1st, 2nd, and 3rd pay zones) and fractured shales (4th zone). The sandstone zones form typical stratigraphic traps, because they are isolated sandstone bodies within a thick formation which is essentially shale. The structural geometry appears to be irrelevant. The discovery well location, on what was thought to be the axis of a NE.-SW.-trending anticline, was based on surface geology and seismic data. The well was completed as an oil producer (75 bbl or 12 cu m/day) in sandstone lenses of the Candeias Formation (Cretaceous).

Use of Gravity, Magnetic, and Electrical Methods in Stratigraphic-Trap Exploration: Geophysical Exploration Methods

Abstract: Gravity, magnetic, and electrical methods of prospecting can be useful in exploration for pinchout and reef traps. A pinchout zone might produce a recognizable gravity anomaly as a result of the density contrast (1) between the porous sandstone and the adjacent beds or (2) between the water-saturated and oil-saturated parts of the sandstone section. Reefs also can produce gravity anomalies as a result of differences in density between the reef material and the laterally adjacent beds--whether these beds are salt, limestone, or shale. Where the reef is in shale, compaction of the shale in the area adjacent to and above the reef is an important factor. Magnetic methods are useful where basement structure is a factor in formation of stratigraphic traps. The use of electrical methods in exploring for stratigraphic traps is much less promising. Overall, however, these potential methods have possibilities for application in the search for certain types of stratigraphic traps.

San Emidio Nose Oil Field, California: Case Histories

Abstract: The San Emidio Nose oil field has a reserve of 64 million bbl. The accumulation is in a stratigraphic trap in upper Miocene beds; the trap crosses the plunge of a subsurface anticline. The field was discovered in 1958 by the Richfield Oil Corporation (now Atlantic Richfield Company) after 24 years of exploration by six companies. Geographically, the field is located on the southernmost rim of the San Joaquin Valley. Structurally, it is located in the Maricopa subbasin and is one of several simple folds that were defined by geophysics and drilled during the 1930s. At San Emidio, however, the absence of suitable reservoir rocks on the crest of the structure was partly responsible for delaying the discovery until 1958, and six expensive dry holes were drilled before the sand tone geometry was fully defined. The trap is closely related to the westward thinning of the reservoirs (Reef Ridge and Stevens sandstones) and their enclosing shales up the east plunge of the fold. This thinning, which was shown geophysically prior to the discovery, is an elementary example of the stratigraphic application of geophysics.

Vertical net sandstone determination for isopach mapping of hydrocarbon reservoirs. [Correction for stratigraphic altitude and well bore orientation in the determination of recoverable reserves]

Abstract: The usual procedure for determining net effective sandstone values to be used in reservoir isopach mapping is to count the net sandstone thickness as measured by logging devices in each well bore penetrating the reservoir sandstone, without applying possible corrections for nonvertical penetration. Application of this procedure is a practical approach to the determination of net sandstone thickness, and the values obtained probably are within the limits of accuracy needed for most reservoir isopach maps. However, certain conditions of bed dip and/or bore-hole deviation preclude the use of this procedure as an accurate method for net sandstone thickness determination. The difference between measured and vertical net sandstone thickness can be significant. As an example, wh re reservoir bed dip is 60° and a well bore drifts directly updip with a 7° angle of inclination, the true vertical net sandstone thickness attributable to the well is 20 percent greater than that observed in the well bore. The use of vertical net sandstone values in reservoir isopach mapping results in a more nearly accurate estimate of recoverable reserves.

West Campbell (Northeast Cedardale) Gas Field, Major County, Oklahoma: Case Histories

Abstract: The West Campbell field is a prime example of the principal type of Hunton stratigraphic trap in the Anadarko basin. Gas and condensate are produced from a lens of dolomite within the predominantly dense limestone of the Hunton Group of Silurian-Devonian age. The field was discovered by drilling on a seismic anomaly with unproved north closure in an area where subsurface control indicated the presence of porous and permeable Hunton dolomite. Later drilling proved that the trap was stratigraphic. Because the Hunton hydrocarbons are trapped almost entirely within the dolomite facies, it is essential in exploration for further traps of this type to relate the origin of the dolomite members to the history of deposition, diagenesis, and oscillations of sea level in a developed field such as West Campbell. As evidenced by various minor unconformities in localized areas, the sea oscillated frequently during the time of Hunton deposition. It is believed that, at each period of prolonged stability of the sea, dolomite was deposited all along the shoreline in shallow water. As a result, several trends of dolomite parallel the old Hunton shoreline.

Sergnano Gas Field, Po Basin, Italy--A Typical Stratigraphic Trap: Case Histories

Abstract: Gas accumulation in stratigraphic traps discovered to date in the Po basin is limited to the "Pede-Alpine" region. The Sergnano field is a clear example of a stratigraphic trap. The gas reservoir is a sand-gravel body, closed by abrupt facies variation into lowermost Pilocene shale; a transgressive basal conglomerate of Pliocene age overlies truncated lower Miocene-Oligocene strata. Reflection seismic surveys were the determining factor in the discovery of this kind of trap. However, a contribution to the detection of similar but less evident stratigraphic traps was made by a device developed in the research department of AGIP Mineraria.

Gas injection with radioactive tracer to determine reservoir continuity, East Coalinga field, California

Abstract: The Temblor Zone II reservoir consists of intervals of movable oil associated with intervals of high gas saturation or desaturated intervals. Natural gas injection into these desaturated intervals, using tritium and krypton as radioactive tracers has served to determine reservoir continuity. In these example cases, the desaturated intervals contained nearly all carbon dioxide gas. The injection tests also have furnished data concerning the nature and distribution of desaturated intervals in an unconsolidated sand reservoir with permeabilities of 300 to 500 md and porosity of 26% having 21/sup 0/ API gravity oil with a viscosity of 25 cp at reservoir temperature. It can be concluded from these tests that (1) the injection of radioactive gas into reservoirs having oil desaturation can be used to map continuity, (2) gas-oil interfaces found in some gravity drainage reservoirs deviate from the horizontal, and (3) the gas overlying the movable oil will likely influence supplemental recovery projects by acting as a thief zone for injected water.