Matrix-related pore networks in mudrocks are composed of nanometer- to micrometer-size pores. In shale-gas systems, these pores, along with natural fractures, form the flow-path (permeability) network that allows flow of gas from the mudrock to induced fractures during production. A pore classification consisting of three major matrix-related pore types is presented that can be used to quantify matrix-related pores and relate them to pore networks. Two pore types are associated with the mineral matrix; the third pore type is associated with organic matter (OM). Fracture pores are not controlled by individual matrix particles and are not part of this classification. Pores associated with mineral particles can be subdivided into interparticle (interP) pores that are found between particles and crystals and intraparticle (intraP) pores that are located within particles. Organic-matter pores are intraP pores located within OM. Interparticle mineral pores have a higher probability of being part of an effective pore network than intraP mineral pores because they are more likely to be interconnected. Although they are intraP, OM pores are also likely to be part of an interconnected network because of the interconnectivity of OM particles. In unlithifed near-surface muds, pores consist of interP and intraP pores, and as the muds are buried, they compact and lithify. During the compaction process, a large number of interP and intraP pores are destroyed, especially in ductile grain-rich muds. Compaction can decrease the pore volume up to 88% by several kilometers of burial. At the onset of hydrocarbon thermal maturation, OM pores are created in kerogen. At depth, dissolution of chemically unstable particles can create additional moldic intraP pores.

Spectrum of pore types and networks in mudrocks and a descriptive classification for matrix-related mudrock pores

Transport of suspended participate matter is widely recognized to occur in subsurface environments. Field data indicate that viruses, bacteria, and clay minerals can migrate considerable distances and that small particles and macromolecules are implicated in the transport of organic contaminants and radio-nuclides. Furthermore, media permeability can be significantly altered by changes in aqueous chemistry through particle release and capture. Quantitative models for predicting particle transport are available within the water filtration literature that account for the mechanisms of particle-media collisions and the conditions for attachment. Predictions from the filtration models are used to analyze particle migration through porous media at typical groundwater flow velocities. As particles accumulate within media pores, available models become less predictive because of the coupling between particle retention and permeability reduction. An examination of filtration data reveals that retention of a relatively small solid volume within media pores can reduce media permeability by orders of magnitude. The fact that contaminants adsorbed to particles are mobile has important implications in understanding and predicting contaminant transport. The design of laboratory experiments and the collection of field samples often neglect contaminants transported by suspended colloids and particles.

Particle transport through porous media

Organic-carbon-rich strata or "black shales," including dark gray to black, laminated, carbonaceous mudrocks characterized by impoverished benthonic faunas, or devoid of metazoan life, have long intrigued geol­ ogists, both because of their widespread distribution at certain times in the past and their early recognition as potential hydrocarbon source rocks. One of the major obsessions of many early workers, to the mid1900s, was the application of uniformitarian principles to depositional models for black shales. Thus, although it was recognized that some black shale units were unusually widespread, many early workers sought less extensive, modern analogues for environments of formation of dark-colored, rela-

MARINE BLACK SHALES: Depositional Mechanisms and Environments of Ancient Deposits

Diagenesis exerts a strong control on the quality and heterogeneity of most clastic reservoirs. Variations in the distribution of diagenetic alterations usually accentuate the variations in depositional porosity and permeability. Linking the types and distribution of diagenetic processes to the depositional facies and sequence-stratigraphic framework of clastic successions provides a powerful tool to predict the distribution of diagenetic alterations controlling quality and heterogeneity. The heterogeneity patterns of sandstone reservoirs, which determine the volumes, flow rates, and recovery of hydrocarbons, are controlled by geometry and internal structures of sand bodies, grain size, sorting, degree of bioturbation, provenance, and by the types, volumes, and distribution of diagenetic alterations. Variations in the pathways of diagenetic evolution are linked to (1) depositional facies, hence pore-water chemistry, depositional porosity and permeability, types and amounts of intrabasinal grains, and extent of bioturbation; (2) detrital sand composition; (3) rate of deposition (controlling residence time of sediments at specific near-surface, geochemical conditions); and (4) burial thermal history of the basin. The amounts and types of intrabasinal grains are also controlled by changes in the relative sea level and, therefore, can be predicted in the context of sequence stratigraphy, particularly in paralic and shallow marine environments. Changes in the relative sea level exert significant control on the types and extent of near-surface shallow burial diagenetic alterations, which in turn influence the pathways of burial diagenetic and reservoir quality evolution of clastic reservoirs. Carbonate cementation is more extensive in transgressive systems tract (TST) sandstones, particularly below parasequence boundaries, transgressive surface , and maximum flooding surface because of the abundance of carbonate bioclasts and organic matter, bioturbation, and prolonged residence time of the sediments at and immediately below the sea floor caused by low sedimentation rates, which also enhance the formation of glaucony. Eogenetic grain-coating berthierine, odinite, and smectite, formed mostly in TST and early highstand systems tract deltaic and estuarine sandstones, are transformed into ferrous chlorite during mesodiagenesis, helping preserve reservoir quality through the inhibition of quartz cementation. The infiltration of grain-coating smectitic clays is more extensive in braided than in meandering fluvial sandstones, forming flow barriers in braided amalgamated reservoirs, and may either help preserve porosity during burial because of quartz overgrowth inhibition or reduce it by enhancing intergranular pressure dissolution. Diagenetic modifications along sequence boundaries are characterized by considerable dissolution and kaolinization of feldspars, micas, and mud intraclasts under wet and warm climates, whereas a semiarid climate may lead to the formation of calcrete dolocrete cemented layers. Turbidite sandstones are typically cemented by carbonate along the contacts with interbedded mudrocks or carbonate mudstones and marls, as well as along layers of concentration of carbonate bioclasts and intraclasts. Commonly, hybrid carbonate turbidite arenites are pervasively cemented. Proximal, massive turbidites normally show only scattered spherical or ovoid carbonate concretions. Improved geologic models based on the connections among diagenesis, depositional facies, and sequence-stratigraphic surfaces and intervals may not only contribute to optimized production through the design of appropriate simulation models for improved or enhanced oil recovery strategies, as well as for CO2 geologic sequestration, but also support more effective hydrocarbon exploration through reservoir quality prediction.

The impact of diagenesis on the heterogeneity of sandstone reservoirs: A review of the role of depositional facies and sequence stratigraphy

Experiments on the role of water in petroleum formation

Amorphous silica phases (opal-A) precipitate in nature due to the formation of dense colloids in supersaturated alkaline aqueous solutions with low relative concentrations of other ions. Opal-A dissolves and yields solutions of still relatively high silica content. In pore waters containing abundant cations, open framework polymers form which flocculate to yield opal-CT. Opal-CT becomes increasingly ordered, primarily due to preferential growth of cristobalite relative to tridymite and crystal size increase. Opal-CT dissolves to yield pore waters of low silica concentration, which allows slow growth of quartz crystals from monomeric solution. The quartz crystals then slowly increase in size and crystallinity. Carbonates appear to enhance opal-CT formation, possibly due to the activity of positively charged hydroxyl complexes. Hence, polymerization in relatively pure systems is involved in opal-A formation and polymerization in impure systems is involved in opal-CT formation, and slow growth from monomeric (low silica concentration) solutions is involved in precipitation of quartz in sedimentary realms.

Silica diagenesis; II, General mechanisms

The diagenetic sequence of silica is examined in terms of several thermodynamic and kinetic concepts. The relationship between solubility and surface area or particle size is sufficient to explain simple opal-A opal-CT quartz transformations, suggesting that the role of temperature and other physical parameters may not be as critical as previously thought. Complexation and adsorption in an impure system, when added to the model, allow for possibilities of opal-A transformation directly to quartz, of clay and zeolite formation from an opaline silica source, and of variations in silica transformation rates due to presence of impurities in the system.

Silica Diagenesis, I. Solubility Controls

Bacterial mats play an important role in the sedimentation history of many marine biogenic regimes. Observations of the living mats suggest that in shallow waters they are dominated by photosynthetic cyanobacteria, while in deeper waters the mats are dominated by nonphotosynthetic sulfur-oxidizing bacteria. Fossilized remains of analogous mat structures from the Miocene Monterey Formation are commonly associated with oil-producing and phosphate-rich zones. Preliminary observations suggest that bacterial mats may be a source of the organic matter that in time is thermally altered to hydrocarbons.

Role of bacterial mats in oxygen-deficient marine basins and coastal upwelling regimes: Preliminary report

The Miocene Monterey Formation contains thin organic-rich laminations that mimic benthic mats forming today in coastal upwelling regimes of the eastern Pacific. Modern mats are comprised dominantly of filamentous sulfur-oxidizing bacteria of the family Beggiatoaceae. The characteristic spongy texture imparted to the sediments by these bacterial mats is most readily recognized using scanning electron microscopy (SEM). The spongy network becomes progressively compressed with increasing compaction and diagenesis, but is still distinguishable. Rock-Eval pyrolysis of modern mats reveals high S1 peaks and unusually high oxygen indices. Both parameters decrease gradually with diagenesis, as evidenced by mat-laminated Monterey Formation samples of different thermal grades. In addition, anomalously light nitrogen isotopic signatures of modern mats may be retained. The ^dgrN15 values of mat-laminated Monterey Formation samples, although diagenetically enriched in 15N, are still lighter than would be expected for algally dominated organic matter in marine low-oxygen environments. On the basis of organic richness and high hydrogen indices of mat-laminated samples, and coincidence of mat-laminated rocks and oil in some fields in the San Joaquin Valley of California, the organic-matter assemblage contained within filamentous bacterial mats is suggested to be a significant contributor to petroleum formation. Additional evidence for this conclusion is provided by similar investigations of subtidal cyanobacterial mats in carbonate rocks such as the Green River Formation oil shales.

Subtidal stromatolites in Monterey Formation and other organic-rich rocks as suggested source contributors to petroleum formation

The Carboniferous Bear Gulch Limestone of the central Montana Big Snowy Trough is a lithographic limestone analogous to the plattenkalk carbonates of the Mesozoic European/Middle Eastern Tethyan belt. It contains an excellently preserved chitinous and phosphatic marine fauna.



Petrographic and field studies synthesized with palaeontological data provide insights into depositional history. Tectonic activity initiated development of small en echelon basins. Rims of these basins were sites of high primary productivity. Photosynthesis by subtidal cyanobacterial mats and algae caused in situ precipitation of micrite. Cyanobacteria and algae also provided a food source for benthic and nektonic organisms, which provided food for predators. Respiration in turn provided carbon dioxide for continued photosynthesis. Thus a balanced ecosystem evolved.



Productivity in the water column used up available oxygen, resulting in dysaerobic to anaerobic conditions in the bottom waters and sediments. This plus high sedimentation rates of carbonate ooze transported downslope, aided in preservation of whole-body fossils.



There are no likely modern analogues known for the Bear Gulch Limestone. However, comparisons with Mesozoic plattenkalke demonstrate that it is most closely analogous to those deposited in shallow water back-reef areas of the Tethyan carbonate platform, rather than to those deposited in the deeper water shelf-edge environment. This is in agreement with the water depth of roughly 40 m estimated for the Bear Gulch Limestone.

Loretta Ann Williams

Papers

Silica diagenesis; II, General mechanisms

Silica Diagenesis, I. Solubility Controls

Role of bacterial mats in oxygen-deficient marine basins and coastal upwelling regimes: Preliminary report

Subtidal stromatolites in Monterey Formation and other organic-rich rocks as suggested source contributors to petroleum formation

Deposition of the Bear Gulch Limestone: a Carboniferous Plattenkalk from central Montana