Shirley P. Dutton

Calcite cement in Permian deep-water sandstones, Delaware Basin, west Texas: Origin, distribution, and effect on reservoir properties

Calcite cement is the dominant control on reservoir quality in turbidite sandstones of the Upper Permian Bell Canyon Formation, Delaware Basin. These well-sorted, very fine-grained arkoses were deposited in a basin-floor setting by channel-levee systems terminating in broad lobes. Calcite cement distribution in the East Ford and Geraldine Ford fields was mapped using core, log, and thin-section data. Calcite is concentrated in tightly cemented zones that are mostly less than 1 ft (0.3 m) thick. Areas that have high percentages of calcite-cemented sandstone (20%) occur along the margins of the sandstone bodies, in overbank and lobe deposits, where sandstone pinches out into siltstone. Areas that have the lowest percentage of calcite-cemented sandstone (10%) occur where the sandstone is thickest, in the channel facies. Isotopic composition of the calcite (13C = 1.8 to 3.0 [relative to the Peedee belemnite, PDB], 18O = 4.6 to 6.3 [PDB]) is consistent with the source of calcium carbonate being from dissolution of detrital carbonate rock fragments and marine skeletal debris. Because internal sources of calcite were apparently insufficient to account for the cement volume, cement components are interpreted as having been transported into the sandstones from organic-rich basinal siltstones and limestones. Feldspars buffered acidic formation waters near where they entered the sandstone, resulting in calcite concentrated near the sandstone margins. The calcite formed near maximum burial depths of 4800 ft (1.5 km) and temperatures of 104F (40C) from marine pore waters with 18O of approximately 0 (relative to standard mean ocean water). Most of the calcite-cemented zones are interpreted as being concretions that extend no more than a few meters laterally. Production data and geophysical log correlations suggest that some cemented zones are laterally continuous at least 1000 ft (300 m) and cause vertical reservoir compartmentalization. Laterally extensive calcite layers may be associated with the base of turbidite deposits.

Evolution of porosity and permeability in the Lower Cretaceous Travis Peak Formation, East Texas

Porosity and permeability decrease significantly with depth in low-permeability, gas-bearing Travis Peak sandstones on the western flank of the Sabine arch, east Texas. The Travis Peak Formation is approximately 2000 ft (610 m) thick and is buried to depths of 6000-10,000 ft (1830-3050 m). Reservoirs consist chiefly of sandstones of fluvial and paralic origin. Thin-section and core-analysis data from a total of 3680 ft (1122 m) of core from 26 wells indicate that both depositional environment and diagenetic history influenced the present distribution of porosity and permeability. Porosity in clean sandstones decreases from an average (geometric mean) of 16.6% at 6000 ft (1830 m) to 5.0% at 10,000 ft (3050 m). Permeability is reduced four orders of magnitude over the same depth range, from an average (geometric mean) of 10 md at 6000 ft (1830 m) to 0.001 md at 10,000 ft (3050 m). Loss of porosity by mechanical compaction took place mainly within the first 3000 ft (900 m) of burial, prior to significant cementation. The observed decline in porosity and permeability with depth between 6000 and 10,000 ft (1830 and 3050 m) results from (1) increasing quartz cement, (2) decreasing secondary porosity, and (3) increasing overburden pressure that closes narrow pore throats. At any given depth, average permeability is 10 times higher in clean fluvial than in clean paral c sandstones. Clean fluvial and paralic sandstones contain similar volumes of quartz cement, but paralic sandstones are finer grained and contain an average of 7% more total cement.

Calcite cement distribution and its effect on fluid flow in a deltaic sandstone, Frontier Formation, Wyoming

Precipitation of extensive calcite cement during burial diagenesis can strongly modify the depositional permeability of a sandstone reservoir and affect fluid flow during production. To predict subsurface flow through cemented reservoirs, permeability distributions used in fluid-flow models must reflect this diagenetic overprint. Calcite cements in sandstones commonly occur as irregularly distributed concretions, which makes it difficult to predict diagenetic permeability modifications in the subsurface from typically spaced wells. Outcrops can provide a continuous image of heterogeneity produced by concretionary calcite cements. The size and distribution of calcite concretions were mapped in outcrops of the Frewens sandstone, Frontier Formation, in central Wyoming. Large, tabular calcite concretions in this deltaic sandstone generally follow basinward-inclined bedding. Median thickness of the concretions is 0.6 m, length is 4.2 m, and width is 5.3 m. The highest cement fraction is in the high-permeability facies at the top of the sandstone body. Concretion centers are approximately Poisson distributed within the sandstone. The upward-increasing cement fraction is caused by upward-increasing concretion size. Lateral variation in the fraction of the sandstone cemented by calcite has a normal distribution, with a mean of 12% (s = 5%). Spatial distribution of calcite cement in the Frewens sandstone was modeled using indicator geostatistics. Variograms were inferred from outcrop maps of cement. Indicator semivariograms of cement have a range of 30 m horizontally and 2.5 m vertically, dimensions that correspond approximately to the size of the largest concretions. Stochastic images of cement were created using indicator simulation with vertically varying cement proportion. Flow models indicate that concretions make flow paths more tortuous and retard flow in the coarser facies near the top of the sandstone. The fastest path through the sandstone is in the lightly cemented, high net-to-gross center of the sandstone body. Because the cement mainly occurs within the highest permeability facies in the sandstone body, a model based on depositional facies alone would overestimate upscaled permeability of the Frewens sandstone.

Meteoric Burial Diagenesis of Pennsylvanian Arkosic Sandstones, Southwestern Anadarko Basin, Texas

Pennsylvanian arkosic sandstones at Mobeetie field were deposited in fan deltas that prograded onto a shallow shelf in the southern Anadarko basin. The detrital minerals of the sandstones reflect the composition of the Precambrian granites and granodiorites that were exposed in the nearby Amarillo uplift. Distal margins of some fan-delta lobes were reworked by marine processes, and carbonate fossil fragments and oolites were mixed with terrigenous clastics. The diagenetic history and, hence, reservoir quality of the distal, marine-reworked sandstones differ from those of the more proximal, nonreworked sandstones. The earliest diagenetic events--formation of chlorite ooids and precipitation of fibrous submarine Mg-calcite cement--took place in the depositional environment of the marine-reworked sandstones. On the basis of isotopic data, the remainder of the diagenesis evidently occurred in fluids of meteoric origin. Shallow, early meteoric diagenesis induced precipitation of iron-poor calcite spar in reworked sandstones; nonreworked sandstones lack calcite spar but contain early pore-lining chlorite cement. Dissolution of aragonite by fresh meteoric ground water created moldic porosity within reworked sandstones. As burial increased, porosity in both the reworked and nonreworked fan-delta sandstones was reduced by precipitation of authigenic quartz, feldspar, kaolinite, Fe-calcite, and ankerite. Oxygen isotope data suggest that these cements also precipitated from fluids of meteoric origin that became increasingly hot but remained relatively constant in isotopic composition. The last diagenetic events were influenced by the overlying Permian evaporites. Sodium-rich fluids caused partial albitization of detrital plagioclase, and sulfate derived from the evaporites precipitated as anhydrite and celestite.

Reservoir characterization of a Permian deep-water sandstone, East Ford field, Delaware Basin, Texas

Deep-water sandstones of the Delaware Mountain Group in west Texas and southeast New Mexico contained an estimated 1.8 billion bbl of original oil in place, but primary recovery from these fields is commonly less than 20%. East Ford field in Reeves County, Texas, which produces from the Ramsey sandstone in the upper Bell Canyon Formation, went directly from primary production to tertiary recovery by CO2 flooding. Field production has increased from 30 to more than 185 BOPD. Oil recovery has been improved by the CO2 flood, but not as much as expected. Geologic heterogeneities such as interbedded siltstones are apparently influencing reservoir displacement operations in the East Ford unit.A depositional model of the East Ford unit was developed using data from Bell Canyon outcrops and subsurface data. The Ramsey sandstones were deposited by turbidity currents in a basin-floor setting. The sandstones are interpreted as having been deposited in a channel-levee system that terminated in broad lobes; overbank splays filled topographically low interchannel areas. Injection wells located in splay sandstones apparently have poor communication with wells in channel sandstones, perhaps because communication is restricted through levee and channel-margin deposits. The south part of the unit is responding well to the flood because the injection and production wells are in the same interconnected lobe depositional environment.

CEMENTATION AND BURIAL HISTORY OF A LOW-PERMEABILITY QUARTZARENITE, LOWER CRETACEOUS TRAVIS PEAK FORMATION, EAST TEXAS.

Fine-grained quartzarenites (and subar-koses) in the Lower Cretaceous Travis Peak Formation were extensively modified during burial diagenesis. Timing of diagenetic events can be constrained by combining petrographic and geochemical data with data on subsidence and thermal history. The geothermal gradient in the East Texas study area is 38 °C/km now, but it may have been as high as 44 °C/km when the Travis Peak was deposited because of elevated heat flow caused by crustal stretching associated with rifting of the Gulf of Mexico. Illite rims and dolomite were the first authigenic minerals to precipitate in Travis Peak sandstones. Dolomite probably formed soon after deposition at about 25 °C from water with a δ 18 O composition near SMOW. Next, extensive quartz cement, averaging 17% of the rock volume in well-sorted sandstones, occluded much of the primary porosity. The average δ 18 O composition of the quartz overgrowths indicates that they precipitated from meteoric fluids at temperatures of between 55 and 75 °C, at depths of 1 to 1.5 km. Dissolution of orthoclase and albitization of plagioclase followed quartz cementation and occurred prior to mid-Cretaceous movement of the Sabine Uplift. Illite, chlorite, and ankerite precipitated after feldspar diagenesis. The ankerite may have precipitated over a range of temperatures from about 80 to 125 °C, from fluids with δ 18 O composition of about +2‰ (SMOW); +2‰ is the average present composition of Travis Peak water. Most diagenesis ended when oil migrated into the Travis Peak. Later de-asphalting of the oil by solution of gas filled much of the remaining porosity with reservoir bitumen in some zones near the top of the formation.

History of Quartz Cementation in the Lower Cretaceous Travis Peak Formation, East Texas

ABSTRACT Quartz is the most abundant authigenic mineral in sandstones of the Travis Peak Formation in East Texas, but the extent of quartz cementation varies widely. To determine major controls on quartz cement distribution, this study used 431 thin sections from 1,100 m of core from 26 wells. Volume of detrital clay matrix is the main textural control on quartz cement. In sandstones lacking detrital clay, no significant correlation exists between quartz cement and grain size, sorting, bed thickness, or sedimentary structure. Depositional facies was an indirect control on quartz cement distribution because facies controlled detrital matrix content, but no significant difference exists between average quartz cement volume in matrix-free paralic (16.7%) and matrix-free fluvial (17.1%) sandstones Quartz cement increases significantly with present depth from an average of 14.6% at 1,825 m to 20.0% at 3,050 m. This trend of increasing quartz cement is interpreted to be the result of 1) an early episode of quartz cementation by meteoric water that resulted in relatively uniform distribution of quartz within the formation, and 2) a later episode related to the development of stylolites that preferentially added quartz cement in the deepest sandstones. Approximately 10 to 20% of the authigenic silica in Travis Peak sandstones was derived internally from intergranular pressure solution and stylolitization; the remaining 80% was imported from unknown sources outside the sandstones.

Pennsylvanian-Early Permian Depositional Systems and Shelf-Margin Evolution, Palo Duro Basin, Texas

The Palo Duro basin of the Texas Panhandle is filled primarily with Pennsylvanian, Permian, and Triassic strata that record the depositional history of a shallow cratonic basin. Regional deformation during Early Pennsylvanian time across a belt encompassing the southern Oklahoma and Delaware aulacogens resulted in the formation of the basin. Rapid basin subsidence and marine transgression dominated Pennsylvanian depositional history but was followed by marine regression and rapid filling of deeper parts of the basin during Early Permian (Wolfcampian) time. Pennsylvanian and Lower Permian strata consist of four major facies assemblages or depositional systems. An extensive fan-delta system is composed of arkosic sandstones eroded from Precambrian highlands flanking the bas n and deposited by braided streams along the margins of the basin. In the southeastern part of the Palo Duro basin, westward prograding, high-constructive deltas dispersed sediment across a shelf environment and into basin and slope environments. A thick, massive sequence of limestone accumulated seaward of the deltas in a carbonate-shelf and shelf-margin system that encircled most of the basin. The slope and basin system consists of terrigenous clastics that were funneled downslope into deep basinal environments by feeder channels that formed along shelf margins. Interplay between basin subsidence and local sedimentological controls determined the depositional style and resulting facies patterns. Rapid Pennsylvanian subsidence combined with invasion and deposition of terrigenous clastics across carbonate-bank environments caused parts of the northwestern shelf margin to retreat westward toward shallow, clear water. During Early Permian time subsidence rates slowed and sedimentologic controls dominated basin evolution. Thick slope wedges, which were formed by deltas that prograded to shelf edges and debouched sediment into the slope environment, created shallow foundations for subsequent carbonate-bank development and progradation. Porous shelf-margin dolomites, delta-front sandstones, and fan-delta arkoses are considered potential reservoirs for oil and gas. Potential source rocks may be present in adjacent, thick basinal and slope shales.

Outcrop characterization of reservoir quality and interwell-scale cement distribution in a tide-influenced delta, Frontier Formation, Wyoming, USA

Abstract Petrographic study of the Frewens sandstone, Upper Cretaceous Frontier Formation, documents reservoir-scale diagenetic heterogeneity. Iron-bearing calcite cement occurs as large concretions that generally follow bedding and are most common near the top of the sandstone. Median thickness of the concretions is 0.6 m, length 4.5 m, and width 5.7 m; median volume is 5.2 m3. Concretions comprise 12% of the sandstone. The minus-cement porosity of concretion samples is low, indicating that the calcite precipitated near maximum burial depth. Isotopic and burial history data suggest that the calcite precipitated at ~54°C from evolved meteoric water enriched in 18O or from a mixed meteoric±marine pore-water. Shell-bearing transgressive shales above the Frewens sandstone are interpreted to be the source of calcium carbonate. Concretions of this size and distribution would influence fluid flow in a reservoir and would reduce the amount of hydrocarbons in place.

Influence of Provenance and Burial History on Diagenesis of Lower Cretaceous Frontier Formation Sandstones, Green River Basin, Wyoming

ABSTRACT The Upper Cretaceous Frontier Formation on the Moxa Arch in the western Green River Basin, Wyoming, has had a varied diagenetic history that was controlled in part by differences in composition of detrital framework grains and in burial history. Petrographic examination of 247 thin sections from 13 cores from the south-plunging arch and adjacent deep basin is the basis for diagenetic investigation of sandstones ranging in depth from 2 km to almost 5 km. Major diagenetic events were (1) mechanical compaction by grain rearrangement and deformation of ductile grains, (2) formation of illite and mixed-layer illite-smectite rims, (3) precipitation of quartz overgrowths, (4) precipitation of calcite cement, (5) generation of secondary porosity by dissolution of feldspar, chert, biotite, and mudstone grains and calcite cement, (6) precipitation of kaolinite in primary and secondary pores, and (7) chemical compaction by intergranular pressure solution and stylolitization and additional precipitation of quartz cement. The northern and southern ends of the Moxa Arch differ in the magnitude of each of these diagenetic events. Provenance differences caused more abundant ductile rock fragments and feldspar to be deposited at the northern end of the Moxa Arch. As a result, Frontier sandstones from the northern Moxa Arch underwent more extensive mechanical compaction. In addition, feldspar dissolution and albitization buffered acid-rich basinal fluids at the northern end, resulting in greater development of secondary porosity and precipitation of calcite cement than at the southern end. Deeply buried sandstones at the southern end of the arch and in the basin contain the most abundant quartz cement because intergranular pressure solution and stylolitization liberated silica for overgrowths.