Abstract
Abstract Field mapping, facies analysis, and laboratory study have been completed on Upper Devonian to Lower Permian carbonates of the Bolshoi Karatau Mountains, southern Kazakhstan, and of the southern Urals, Russia. These data suggest that: (1) following the Late Devonian extinction event, major shifts took place in the composition and spatial distribution of reef biota and in the mineralogy and stable- isotope composition of abiotic marine precipitates, and (2) the distribution of reservoir-grade porosity is strongly influenced by early-marine cementation, dolomitization, and meteoric-related diagenetic processes. Corresponding shifts in the morphology of carbonate platforms, and the spatial distribution of potential reservoirs, suggest a strong biotic and abiotic influence on platform architecture, cementation patterns, and reservoir development. Importantly, results of this study describe a series of depositional models and associated diagenetic patterns that are comparable to the known distributions of reservoirs in giant carbonate fields of similar age in the subsurface of the North Caspian basin. Upper Devonian (Fammenian) reefs are composed of Girvanella boundstones, algal cementstones, and brachiopod bioherms and form reef-rimmed platforms. Boundstone and slope deposits are cemented by fibrous and radiaxial fibrous calcite; inner-shelf peritidal deposits are extensively dolomitized, and several intervals of karst are present near the Fammenian-Tournaisian boundary. Tournaisian to Lower Visean carbonates lack a defined organic margin and form ramps that contain Waulsortian mud mounds; inner-shelf peritidal facies are partly dolomitized. Middle-Late Visean and Serpukhovian carbonates form rimmed platforms consisting of peritidal shelf lithologies, grainstone margins, resedimented carbonate and microbial boundstone upper slopes, and deep slope mounds. Shelf interiors consist of cyclic, meter-scale peritidal carbonate lithologies that are capped by paleosols and contain intervals of carbonate-clast conglomerates that are interpreted as forming through prolonged periods of exposure. These layered shelf carbonates are both dolomitized and karsted, and their sedimentation and early diagenesis is attributed to climate changes brought about by the onset of glaciation during the Middle-Late Visean. Slope mounds are composed of sponge-bryozoan-Tubiphytes-algae boundstone and cementstone. Radiaxial fibrous calcite infills skeletal growth frameworks and hash layers, and forms thick linings in stromatactis cavities. Thus mound stabilization is achieved through void cementation and the mantling of mud-dominated fabrics. Bashkirian shelf margins consist of donezellid-brachiopod boundstone and cementstone; shelf interiors contain prolific oolitic grainstone. Bashkirian ooids and marine cement were dominantly composed of aragonite and were particularly susceptible to exposure-related diagenesis and indicate that shelf and margin sediments were exposed during lowstands. Lower Permian shelf margins and slope mounds are composed of bryozoan-Tubiphytes-algae boundstone and cementstone, Palaeoaplysina rudstone, and foram-rich lithologies. Incomplete infilling of boundstone and rudstone lithologies by radiaxial calcite and subaerial exposure resulted in the preservation of primary growth and shelter cavities. On the basis of detailed field and laboratory study of outcrop, it is concluded that biota, eustasy, primary mineralogy, and diagenesis exerted strong control on platform architecture and reservoir development in thick, Upper Devonian to Lower Permian carbonates. Following a Late Devonian extinction event, platform architecture exhibits a transition from reef-rimmed to ramp to shoal-rimmed to reef-rimmed carbonate platforms. This transition in platform architecture is interpreted to reflect: (1) the initial replacement of reef-forming skeletal organisms by reef- forming algae and maintenance of reef-rimmed platforms (Fammenian), (2) demise of algal-constructed margins concomitant with biotic recovery and formation of ramps (Tournaisian-Lower Visean), (3) deposition of microbial boundstone in upper slopes and early stabilization of ooid grainstone to form shoal-rimmed platforms (Middle Visean-Serpukhovian) with mound-building organisms relegated to subphotic slope positions, (4) eventual reestablishment of shallow-water benthos capable of forming wave-resistant platform margins (reef-rimmed platforms; Bashkirian), and (5) formation of reef-rimmed platforms with slope-mound complexes (Lower Permian). Marine cement is particularly abundant in slope mounds, resedimented upper-slope deposits, and reefal boundstone at platform margins, where it occurs as frame-filling cementstone and thick isopachous linings in pores. An association between cementstone and trophic biotic assemblages suggests that massive cementation was enabled by upwelling of carbonate-saturated bottom waters. Isotopic compositions of originally aragonitic and calcitic marine cement are enriched in 13C and 18O and are in agreement with secular trends defined by other Devonian to Permian carbonates. Thus, early marine cementation plays an important role both in constructing these lithofacies and in reducing most primary depositional porosity. As a result, porosity-enhancing diagenetic processes in the slope and platform margin (e.g., early marine- related dolomitization; leaching) are limited to partial alteration of skeletal fabrics, cements, and micritic matrix. Partial marine cementation of shelf-margin grainstone and a relative absence of marine cement in shelf-interior lithologies allows for extensive development of porosity-enhancing diagenetic processes that are related to small-scale and large-scale relative sea-level fluctuations (e.g., glacio-eustasy). Diagenetic processes that have been mapped and identified by petrographic and geochemical study include marine-related dolomitization, mixing-zone dolomitization, and meteoric leaching. In the field, reservoir development is expressed as: (1) early marine-related dolomitization of shelf-margin oolite and interior lagoonal sediments, which results in formation of intercrystalline and moldic porosity, and thin stratal reservoirs at a scale of meters to tens of meters; and (2) karsting of platform-interior lithologies, which results in formation of vugs, molds, and microporosity, and thin and thick stratal reservoirs at the scale of meters to < 100 meters. The observed changeover from Upper Devonian-Tournaisian meter-scale peritidal carbonate cycles to Visean-Serpukhovian meter-scale carbonate cycles capped by paleosols and carbonate-clast conglomerates is interpreted to reflect a changeover from “greenhouse” to “icehouse” conditions during the Middle to Late Visean. Thus, Visean to Serpukhovian shelf carbonates are characterized by sequential meter-scale layering and early meteoric diagenesis, and Bashkirian reservoirs are more susceptible to diagenetic enhancement due to original aragonite composition. The sequential deposition of Upper Devonian through Lower Permian carbonates results in a thick stack of relatively tight lithologies at platform margins and slopes, and porous lithologies within shelf margins and interiors. Comparison of predicted diagenetic patterns and reservoir geometries based on field studies with observed diagenetic and reservoir attributes of Karachaganak and Tengiz fields suggest that similar diagenetic processes were operative in the North Caspian basin.