Mark W. Longman

Carbonate Diagenetic Textures from Nearsurface Diagenetic Environments

Understanding the processes and products of carbonate diagenesis is essential to exploration for, and optimum development of, hydrocarbon reservoirs in carbonate rocks. Much (and perhaps most) cementation and formation of secondary porosity (except fractures) in carbonates occurs at relatively shallow depths in one of four major diagenetic environments: the vadose zone, meteoric phreatic zone, mixing zone, and marine phreatic zone. Each of these zones may be divided into several parts on the basis of rate of water movement and saturation of the water with respect to calcium carbonate. Most carbonates are deposited in and begin their diagenetic history in the marine phreatic environment. This zone may be divided into two end members of a continuous spectrum: a zone of relatively little water circulation in which micritization and minor intragranular cementation occur, and a zone of good water circulation near the sediment/water interface of shelf margins or the upper shoreface in which extensive intergranular and cavity-filling cementation occur. Fibrous aragonite and micritic Mg-calcite are the dominant cements. With subaerial exposure, fresh water will replace sea water in the pores of shallow-water carbonates, and a zone of mixed fresh and marine waters may form. In long-lived mixing zones, dolomite may form if the water is of relatively low salinity, whereas bladed Mg-calcite may form if the water is relatively marine. Active water circulation in the mixing zone, which may be caused by seasonal rainfall, is necessary for dolomitization or cementation. Diagenesis in the freshwater phreatic environment may involve leaching in the zone of solution, neomorphism of grains accompanied by extensive intergranular calcite cementation in the active saturated zone, or neomorphism of grains without cementation in the stagnant saturated zone. Syntaxial overgrowths on echinoderm fragments and interlocking crystals of equant calcite that coarsen toward pore centers are typical of cementation in the active freshwater phreatic zone. The freshwater vadose environment is the zone with both air and meteoric water in the pores and may be divided into the zone of solution and the zone of precipitation. CO2 from the atmosphere and soil contributes to solution which generally occurs near the soil zone and forms vugs, molds, and etched grains. When the water becomes saturated with respect to calcite, evaporation or CO2 loss may cause precipitation of fine equant calcite in the form of pendant and meniscus cements. Grains may be altered to calcite, particularly in humid climates, and caliche crusts may be produced by evaporation and/or biologic (generally algal) factors. Climate plays an important role in early diagenesis if subaerial exposure occurs. In arid climates, cementation in freshwater environments may be limited and primary intergranular porosity may be preserved. In humid climates, little primary porosity is likely to escape cementation, but significant amounts of secondary moldic and vuggy porosity may form. Interpretation of diagenesis in carbonates is complicated by the fact that diagenetic environments may change many times in the history of a carbonate rock. By recognizing the processes leading to the formation or preservation of porosity, and the distribution of diagenetic End_Page 461------------------------------ environments in which those processes acted, the distribution of porosity in subsurface carbonates can often be predicted.

Organic Geochemistry of Mid-Continent Middle and Late Ordovician Oils

Ordovician oils in Mohawkian and Cincinnatian reservoirs of the United States Mid-Continent retain the biochemical imprint of Middle and Upper Ordovician oceanic life before the evolution of land plants and most vertebrates. Thus, these oils have some geochemical features that distinguish them from younger oils. These features include (1) a predominance of n-C15, n-C17, and n-C19 alkanes in the saturated hydrocarbon fraction, (2) relatively low amounts of longer chain n-alkanes, (3) low amounts of chlorophyll-derived isoprenoids, such as pristane and phytane, and (4) abundant C29 sterane relative to C27 with rearranged forms (diasteranes) predominant over normal steranes. Ordovician oils also generally contain little sulfur and have a somewhat variable light stable carbon isotopic composition with ^dgr13Csat and ^dgr13Caro values of -28 to -31^pmil (PDB), but these features are typical of many marine oils. The unusual chemistry of these Ordovician oils supports the interpretation of Reed, Illich, and Horsfield (1986) that prokaryotic organisms provided the organic matter for most Ordovician oils. Although their claim for Gloeocapsamorpha (a problematic unicellular prokaryote, possibly a blue-green alga or an unusually large bacterium) cannot be proven from oil chemistry alone, knowing that indigenous Mid-Continent Ordovician oils were derived from prokaryotic organisms may aid in future exploration for these reservoirs.

Sedimentology and diagenesis of the St. Peter Sandstone, central Michigan Basin, United States

The Middle Ordovician St. Peter Sandstone occurs at depths between 1600 and 3600 m in the central Michigan basin. Integration of conventional core and wireline log studies indicates that the formation consists of, from base to top, (1) up to 250 m of sandstone deposited in intertidal and supratidal sand flats with associated dolomitic lagoonal deposits, and shallow subtidal shoreface environments, (2) sandstones deposited in subtidal shoreface to upper offshore environments, and, (3) dolomitic and argillaceous sands deposited in a storm-dominated epeiric sea. Sandstone composition is closely related to depositional environment: quartz arenites occur in higher energy littoral facies, whereas feldspathic and carbonate-rich sands occur in predominantly lower energy shelfal f cies. Important modification of primary mineralogy and porosity occurred during diagenesis. A generalized, basinwide model for paragenesis includes (1) early calcite marine cement, (2) syndepositional dolomitization, (3) widespread precipitation of quartz and K-feldspar overgrowths, (4) pervasive replacement of early carbonate by burial dolomite, (5) local dissolution of unstable framework grains (e.g., K-feldspar) and carbonate cement, (6) growth of authigenic clay, and (7) pressure solution and additional precipitation of quartz overgrowths. Authigenic clays apparently formed along with or subsequent to economically significant secondary porosity. The different pathways of sandstone diagenesis observed in the St. Peter Sandstone are largely dependent on primary textures and composition inherited from the environment of deposition rather than other factors, such as depth of burial or position in the basin. The relationships among primary textures and composition, diagenesis, and reservoir sandstone properties is useful for prediction of reservoir quality in the St. Peter Sandstone in the Michigan subsurface. Weakly cemented sandstone reservoirs characteristic of high-energy shoreface and shallow offshore depositional environments have the best reservoir quality in the basin. Clay-cemented sandstone reservoirs deposited on offshore shelves have the poorest reservoir quality. Incompletely quartz-cemented sandstone reservoirs, comm n in intertidal/supratidal facies, have intermediate reservoir quality.

Unconformity-related chert/dolomite production in the Pennsylvanian Amsden Formation, Wolf Springs fields, Bull Mountains basin of central Montana

Wolf Springs field (north and south pools) and South Wolf Springs field (a.k.a. Wolf Springs fields), located in Yellowstone County, Montana, were discovered in 1955 and 1957, respectively, and have produced more than 5.7 million bbl oil from the Pennsylvanian Amsden Formation. Amsden reservoir rocks in the area are fractured and brecciated cherts and dolomites that occur in several laterally persistent and mappable zones. The Amsden was deposited in a peritidal to sabkha setting where evaporite minerals, mainly anhydrite, were once common. These evaporites were partly replaced by silica (chalcedony and chert) soon after deposition. Later dissolution of the remaining evaporites soon after the silicification event, or during the pre-Middle Jurassic unconformity (PMJU), produced the solution-collapse chert breccias that now serve as the best reservoir facies in the field. Subtle variations in the diagenetic history of these breccias was a major factor in shaping reservoir quality. The Wolf Springs fields are unconformity-related combination structural and stratigraphic traps. The fields are located on a structural closure on the Custer anticline, where porosity and permeability development exhibit a northeast-southwest orientation perpendicular to structural strike of the anticline. The solution-collapse breccias pinch out laterally into either dense dolomites or anhydrite-plugged collapse breccias. The overlying shaly dolomite breccia of the Botts member (informal name) located just below the Piper unconformity and the Jurassic Piper Limestone provide an effective top seal. (Begin page 132) Understanding the geographic distribution of the chert/dolomite zones provides a key to exploration for these reservoirs. This must be coupled with analysis of available rocks and drillstem-test data and the integration of the regional hydrodynamic forces affecting the Amsden. These exploration tools should lead to the discovery of new Amsden reserves in the Bull Mountains basin and Central Montana platform.

Characteristics of a Miocene Intrabank Channel in Batu Raja Limestone, Ramba Field, South Sumatra, Indonesia

Ramba field in the South Sumatra basin produces oil from lower Miocene coral and foraminifera-rich wackestones with secondary moldic and vuggy porosity. These wackestones form two low-relief carbonate banks separated by a deeper water, intrabank channel at least 10 km long and 1 km wide. Sediments infilling the channel include (1) carbonate mudstone, (2) carbonate conglomerate (calclithite) composed of dolomite and micrite intraclasts, (3) foraminiferal packstone to grainstone, and (4) terrigenous shale rich in planktonic foraminifera. The channel formed during fault-controlled downwarping during (or followed by) growth of the adjacent carbonate banks. Slow rates of carbonate deposition in the clay-rich environment of the channel resulted in deposition of relatively tight carbonates that form a nonproductive barrier between the A and B oil pools in Ramba field. The channel-fill deposits are economically important for two reasons: (1) where tight, they act as a lateral seal to the reservoir, and (2) the carbonate conglomerate has sufficient porosity and permeability to have provided the spillpoint for oil in the A pool. Thus, although not sufficiently permeable to provide commercial production, it is the distribution of this conglomerate in the channel that limits the height of the oil column in the Ramba fields A pool to only 48 m.