Grainstone

Abstract: Standard carbonate facies models are widely used to interpret paleoenvironments, but they do not address how carbonate platforms are affected by relative changes in sea level. An understanding of how the subtidal carbonate "factory" responds to relative sea-level changes and the role played by other environmental factors towards influencing the formation of carbonate platforms allows one to differentiate platform types and it helps establish a basis for constructing depositional sequence and systems tract models. The combination of in-situ production of carbonate sediment, which is also subject to transport, and local variations in depositional processes result in the formation of a wide variety of stratal patterns, some of which are unique to carbonate systems. Fundamental carbonate-depositional principles and geologic-based observations were used to construct depositional sequence and systems tract models for a variety of rimmed shelves and ramps. The models show how, for example, depositional sequences made up of (1) carbonate, (2) carbonate-siliciclastic, or (3) carbonate-evaporite-siliciclastic facies are produced by depositional systems responding to lowstand, transgressive, and highstand conditions. Lowstand: Carbonate sediment production is reduced on rimmed shelves because a relatively small area of shallow seafloor is in contact with the carbonate "factory." Reduced sedimentation and subaerial exposure foster the retreat of shelf edges and slopes by erosion and slope failure during lowstands. As a result, thick debris-flow eposits may form. Karst development is important in humid climates and can affect large areas of a subaerially exposed platform. If siliciclastic sediments are available, they are delivered to the shelf edge and slope by fluvial-deltaic systems or, in arid climates, by wadis and advancing ergs. Under arid conditions, lowstand evaporites may fill an isolated or completely silled basin. Transgression: Carbonate sedimentation initiates in restricted environments and later as more open conditions develop, open marine facies, including patch reefs, may locally develop atop flooded platforms and ramps. Retrogradational End_Page 3-------------------------- parasequences comprising shallow-water carbonates form and subsequently drown, and shelf edges tend to aggrade, backstep, and drown if the rate of sea-level rise is high. Highstand: Seaward-prograding carbonate or siliciclastic coastal sediments and landward-prograding carbonate rimmed shelf edges may partially fill inner to outer shelf seas. Under arid conditions, evaporites and red beds commonly fill wide and shallow salinas. These strata onlap subaerially exposed rimmed shelf edges and prograding grainstone islands in ramps. Shelf edges and shorelines tend to prograde under the influence of high rates of carbonate sedimentation across the shelf and shelf edge. Slope and basinal environments receive excess shelf- and shelf-edge-derived sediment. Factors listed above must be integrated with established facies models in order to arrive at comprehensive sequence and systems tract models. As should be the case with all models, however, they are not meant to serve as rigid templates within which all carbonate sequences must fit. Modification may be needed to accommodate each case. Once they are deemed applicable to a specific case, they function as working hypotheses to help geologists visualize how and why carbonate strata were laid down and fit together as they do. As a general predictor of facies, carbonate depositional sequence and systems tracts models may be used in conjunction with seismic records to identify depositional systems and to locate reservoir-, seal-, and source-prone facies.

Abstract: Stratigraphic, lithologic, and geochemical data provide ample evidence that carbonate rocks and not shales are the source beds of commercial oil in carbonate grainstone reservoirs of the Lower Cretaceous Sunniland Limestone of the South Florida basin. Detailed crude-oil-source-rock correlations, including gas-chromatographic-mass-spectrometric analyses of steranes, and of tricyclic and pentacyclic terpanes, indicate that the algal-sapropelic, organic-rich argillaceous limestones in the lower Sunniland Limestone, particularly the downdip, basinal facies, are the probable major source of upper Sunniland oil. Conventional geochemical maturity parameters, plus biomarker isomerization ratios, indicate that the Sunniland oils and source rocks are marginally mature. The sterane ratios, commonly in conjunction with the extended hopane isomerization ratios, proved invaluable in deciphering the thermal maturity of carbonate rocks in south Florida and in pinpointing the upper threshold of the conventional oil-generation zone.

Abstract: Petroleum production from restricted shelf carbonates of the Lower Ordovician Ellenburger Group is commonly considered to have been a result of a pervasive, relatively homogeneous tectonic fracture system within the reservoir rock. However, regional facies and diagenetic (paleokarst) studies of Ellenburger strata, based on cores and wireline logs, have demonstrated that significant reservoir compartmentalization was caused by karst modification in the upper part of the unit. Ellenburger Group carbonates, which attain a thickness of over 1,700 ft (520 m), record sedimentation on a shallow-water restricted shelf occupying most of west Texas. Logging of over 10,000 ft (3,050 m) of core from 63 wells has allowed recognition of six facies assemblages: (1) lithic arenite, (2) mixed siliciclastic-carbonate packstone-grainstone, (3) ooid-peloid grainstone, (4) mottled mudstone, (5) laminated mudstone, and (6) gastropod-intraclast packstone-grainstone. These facies assemblages record initial transgression and subsequent progradation and aggradation. Paleoslope was generally south and east from the Texas arch toward the Ouachita and Marathon orogenic belts. All facies assemblages, with the local exception of the ooid-peloid grainstone assemblage, are characterized y very low intergranular and intercrystalline porosity. Porosity development in Ellenburger Group carbonates is directly related to a prolonged period of subaerial exposure that coincided with a Middle Ordovician eustatic lowstand prior to transgression of Simpson Group siliciclastics. During this episode, a widespread system of caves, sinkholes, joint-controlled solution features, and collapse breccias developed. Of particular importance to reservoir development was the formation of a regionally extensive cave system between 100 and 300 ft (30 and 90 m) beneath the exposed Ellenburger surface. Infill of this cave system by Simpson Group sand and clay segmented the upper Ellenburger into three karst facies, which are, in descending order, (1) cave-roof dolomites (fracture and mosaic breccias), (2) laterally persistent cave-fill facies (siliciclastic-matrix-supported and carbonate-matrix-supported breccias), and (3) lower collapse facies (chaotic clast-supported breccias) of the cave floor. Pronounced vertical segregation of permeable zones defined by the three karst facies is evident in the Emma, Andector, Martin, Block 13, and several other major Ellenburger reservoirs. Lateral reservoir heterogeneities formed by localized laterally extensive collapse structures, such as in the Shafter Lake reservoir, also contribute to compartmentalization of producing zones within the upper Ellenburger Group. Secondary and tertiary recovery programs in these Ellenburger reservoirs can be optimized by integrating concepts of lateral and vertical heterogeneity predicted by the karst model.

Abstract: Two separate and distinct diamictite-rich units occur in the mixed carbonate- siliciclastic Polarisbreen Group, which comprises the top kilometer of47 km of Neoproterozoic strata in the northeast of the Svalbard archipelago.The platformal succession accumulated on the windward, tropical to subtropical margin of Laurentia.The older Petrovbreen Member is a thin glacimarine diamictite that lacks a cap carbonate. It contains locally derived clasts and overlies a regional karstic disconformity that was directly preceded by a large (410% )n egatived 13 C anomaly in the underlying shallow-marine carbonates.This anomaly is homologous to anomalies in Australia, Canada and Namibia that precede the Marinoan glaciation.The younger and thicker Wilsonbreen Formation comprises terrestrial ice- contact deposits. It contains abundant extrabasinal clasts and is draped by a transgressive cap dolostone 3^18 m thick.The cap dolostone is replete with sedimentary features strongly associated with post-Marinoan caps globally, and its isotopic pro¢le is virtually identical to that of other Marinoan cap dolostones. From the inter-regional perspective, the two diamictite-rich units in the Polarisbreen Group should represent the ¢rst and ¢nal phases of the Marinoan glaciation. Above the Petrovbreen diamictite are 200 m of ¢nely laminated, dark olive- coloured rhythmites (MacDonaldryggen Member) interpreted here to represent suspension deposits beneath shorefast, multi-annual sea ice (sikussak). Above the suspension deposits and below the Wilsonbreen diamictites is ao30-m-thick regressive sequence (Slangen Member) composed of dolomite grainstone and evaporitic supratidal microbialaminite.We interpret this sabkha-like lagoonal sequence as an oasis deposit that precipitated when local marine ice melted away under greenhouse forcing, but while the tropical ocean remained covered due to in£ow of sea glaciers from higher latitudes. It appears that the Polarisbreen Group presents an unusually complete record of the Marinoan snowball glaciation.