Abstract Examination of porosity data from Holocene and Pleistocene carbonate strata indicates that there generally is little or no porosity loss in the zone of near-surface water circulation [that is, in the vadose, meteoric-phreatic, or mixing zone(s)]. Thus, the transition from very porous carbonate sediments to well-cemented, low-porosity carbonate rocks is a dominantly subsurface process. Indeed, both shallow-and deep-marine carbonate strata show a continuous loss of porosity with depth, indicating that porosity-reducing processes act continuously from the surface to depths in excess of 4 km. Experimental, observational, and geochemical data show that porosity loss through burial diagenesis results from both physical and chemical compaction and from cementation. In near-surface sections, dewatering, grain reorientation, grain breakage, and other mechanical processes lead to sediment/rock porosities as low as 30 percent. Continued porosity loss requires mechanical compaction, chemical dissolution at grain contacts and along solution seams or stylolites, and/or re-precipitation of dissolved calcite as intergranular cement. Calcareous shales or marl seams (donor beds) can act as significant sources of dissolved carbonate which is precipitated as cement in adjacent limestones (recipient beds). Quantitantive studies of stylolites and solution seams commonly underestimate the total magnitude of pressure solution because they ignore contributions from associated thick calcareous shale sections and from thinner, regularly and irregularly distributed marl interbeds. Through these mechanisms, carbonate rock porosity may be reduced to values near zero in “semi-closed” systems without significant introduction of allochthonous cementing material. In many young, subsiding basins, patterns of porosity loss with depth are crudely predictable. These patterns provide standards against which individual case studies of diagenesis may be compared and provide predictive tools for estimating porosity prior to drilling. In other areas, the standards allow identification of anomalously high porosity and focus attention on specific mechanisms which would act to preserve primary (or early diagenetic) porosity or to create secondary porosity at depth. Comparisons of oil field porosities with standard curves will allow further refinement of our understanding of diagenetic processes. Predictive models are still in their infancy, however. There is a critical need to independently assess how rates of porosity loss with depth are affected by time, temperature, depositional setting, early diagenetic history, maturation history of organic matter, and other factors. In addition, overpressuring, early oil migration, dolomitization, and hydrothermal alteration are known to affect porosity-depth relationships. Refining our understanding of these factors may help geologists trse their general knowledge of basin history to make valid predictions of carbonate reservoir quality in frontier areas.

The Permian (Leonardian) Bone Spring Limestone is a significant oil reservoir rock in the Delaware Basin of west Texas and southern New Mexico. Analysis of fluid inclusions trapped in fracture-filling cements, oxygen isotope composition of the cements, and thermal maturity measurements of host rock provide a detailed record of the thermal and fluid history for this area of the western Delaware Basin. Although these cements have petrographically distinct multiple zones, homogenization temperatures (T h ) of oil fluid inclusions and δ 18 O data divide these cements into two (early and late) groups. Early cements have δ 18 O near O%c and contain dull- blue fluorescent oil inclusions whose T h is mainly between 55°C and 90°C. The late cements occur in fractures that crosscut the early cements and are strongly oriented subparallel to the local structures formed during the Laramide orogeny and Basin and Range uplift. The late cements have δ 8 O between −8%c and −9%c and contain sparse oil inclusions similar to those in the early cement. Primary fluid inclusions in the late cements have a T h mainly between 90°C and 135°C. Petrographie observations suggest that overlap in T h between early and late cements is due to: (1) incorporation of fragments of early cements into crosscutting late cements; (2) local neomorphism of early calcite along margins of late calcite; and (3) formation of secondary inclusions (some distinguished by yellow fluorescence) during uplift and exposure. When interpreted through burial history reconstruction, T h and δ 18 O data suggest that oil migrated relatively early while this rock was about 55°C to 75°C. These temperatures were attained at near maximum burial depth of 1-2 km (estimated 30°C/km paleogradient) at the end of the Permian. The average T h values of primary inclusions of about 110°C are consistent with maximum temperatures of ca. 120°C interpreted from a mean vitrinite reflectance (R m ) of 0.7%, suggesting that the late cements record the maximum temperature attained in the rock. The change in δ 18 O of about 8%c from the early to late cement is nearly that expected from the difference in T h between these cements. Although maximum burial depth was reached near the end of the Permian, maximum temperature was not recorded until geothermal gradients apparently increased in the Tertiary. The evidence for this higher geothermal gradient is recorded in cements from fractures formed during early Tertiary deformation. Calculations from calcite/water fractionation relations and T h indicate that early calcite cement precipitated from water that had δ 18 O of about +9%c and later cements precipitated from water of about +7.5%c. These values are typical for waters remaining after intense evaporation or for deep basin brines. Temperature determinations are associated with sufficient error that both cement generations may have been precipitated from the same water, and cement isotopic differences may have resulted solely from temperature changes.

Abstract C arbonate sand accumulations of reservoir size commonly occur on or near the seaward edge of banks, platforms and shelves. They may also form within the platform interiors or on topographically high areas in regionally deep water, but these occurrences are not as common as those along the margins. Bank-margin sand accumulations may grade landward or seaward within a fraction of a kilometer of other environments and, thus, do not have wide lateral extent in a dip direction. Such accumulations are sufficiently distinct and economically important as carbonate reservoirs to warrant their treatment in some detail. This paper provides an overview of modern and ancient carbonate sand bodies, and refers the reader to detailed work covering many aspects of carbonate sands. Bank-margin carbonate sands occur repeatedly throughout the geologic record and are a prominent component of carbonate facies models (Fig. 1). Shaw (1964) and Irwin (1965) recognized the persistence of this facies in epeiric sea models as a seaward high-energy zone separating low-energy, deeper-water sediments from low-energy, shallow-water lagoonal deposits. More recent models by Heckel (1972), Lees (1973), and Wilson (1975) emphasize the importance of the zone in which carbonate sands accumulate. In nature, sand accumulations are not distinct and isolated from other facies (as are the chapters of this volume from each other, for example), and there will necessarily be some overlap between this chapter and those concerning reefs, beaches and islands, and lagoons. Our understanding of carbonate sand deposition is biased toward bank-margin deposits because most modern studies have focused on these.

Abstract The Rainbow Lake Field is located some 400 miles northwest of Edmonton, Alberta. The main production comes from Middle Devonian reefs which are encased in evaporites. These reefs vary in size from a quarter section to nine sections and range in height to over 800 feet. Additional production comes from overlying units which are structurally draped over the reefs. These reefs have been interpreted to be ecologic reefs that owe their reservoir qualities to primary fabric and mesodiagenetic processes. To the contrary, we hope to show that these build-ups are “cementation reefs”, i.e. they are primarily products of pervasive cementation of mechanically deposited skeletal debris and that their reservoir characteristics are mainly the result of eodiagenetic processes.

Abstract The Little Knife Field, 12 by 2.5 to 4 miles (19.2 × 4 to 6.4 Km.), lies beneath a broad northward plunging anticline with less than 100 feet (30 m) of structural relief. Production is from early diagenetic porosity in the Mission Canyon Formation. The discovery well flowed 480 BOPD of 43° sour crude from 8′ (2.44 m) of perforations. Cumulative field production from February, 1977 to March, 1980 is over 10,000,000 BO. The Mission Canyon Formation is interpreted as a regressive shoaling upward sequence. It is punctuated by carbonate cycles deposited in deeper and more open marine waters towards its base and shallower and more restricted waters towards its top. The cycles are more difficult to identify up section. The formation may be divided into six informal zones: A) A cap of bedded and nodular anhydrite associated with both a dolomite matrix and laminated interbeds of dolomite whose interpreted depositional setting is supratidal. B) Separated into; 1) an upper portion of interbedded porous, lenticular skeletal wackestones and dense wackestone/grains tones. Interpreted depositional setting of the skeletal wackestones is shallow subtidal while the wackestone/grains tones are part of an intertidal barrier. 2) a lower portion of dolomitized burrowed pelletal wackestone/packstones, which forms a major reservoir. Interpreted depositional setting is restricted shallow marine. C) Dense dolomitized pelletal wackestone with crinoid columns. Depositional settingh is restricted marine to transitional marine. D) Is the other major reservoir zone, and is partially dolomitzed porous burrowed skeletal wackestone and its depositional setting is the seaward portion of a protected shelf and transitional into open marine. E) Dolomitized crinoidal and brachiopod mudstone/wackestone, depositional setting is restricted to transitional marine. F) Alternating skeletal and pelletal packstones and mudstones interpreted as from the open marine. Reservoir beds have also undergone slight to moderate anhydrite replacement of skeletal detritus, which later dissolved.