Walker Creek Field Research Articles

Pore Geometry: Control on Reservoir Properties, Walker Creek Field, Columbia and Lafayette Counties, Arkansas

Walker Creek field in southern Arkansas produces hydrocarbons from oolitic packstones and grainstones of the Jurassic Smackover Formation. The relationships between pore geometry and reservoir quality in these rocks were evaluated using petrographic methods and mercury injection capillary pressure analyses. Results indicate that reservoir quality is controlled by pore geometry, which, in turn, is determined by depositional and diagenetic processes. Reservoir rocks at Walker Creek were deposited as prograding grainstone shoals in a shallow-water, high-energy environment. Diagenetic processes, including early marine cementation, compaction, and deeper burial pressure solution and calcite cementation, modified the original pore system. Primary interparticle porosity is the dominant effective pore type and is most important in terms of reservoir performance. Secondary microporosity is also abundant, comprising a significant percentage of total porosity (locally up to 100%), but is generally ineffective. The rocks were subdivided into lithofacies based on their depositional and diagenetic origin. Within each lithofacies, reservoir, marginal reservoir, and nonreservoir rock types are identified based on their pore geometry and characteristic capillary pressure curve. The reservoir rock types are less cemented oolitic and oolitic-skeletal grainstones with steeply sloping capillary pressure curves and flat plateaus indicating relatively homogeneous pore sizes that are well interconnected by large pore throats. The marginal to nonreservoir rock types include mudstones, wackestones, muddy packstones, and cemented grainstones with generally small interparticle and moldic pores. Gently sloping capillary pressure curves with poorly defined or no plateaus indicate variable pore and pore-throat sizes. A close correlation is found among parts of the capillary pressure curve shape, such as slopes, plateaus, and displacement pressure, and aspects of pore geometry such as pore-throat size, pore-throat size distribution, and pore interconnectivity. In addition, capillary pressure analyses provide guidance in evaluating overall reservoir quality by rock type, as well as answer specific questions regarding net pay criteria as a function of height above free-water level.

Relationship of Pore Geometry to Reservoir Properties: Smackover Formation, Walker Creek Field, Arkansas

Relationship of Pore Geometry to Reservoir Properties: Smackover Formation, Walker Creek Field, Arkansas

Reservoir Description, Walker Creek Field, Arkansas: ABSTRACT

Reservoir Description, Walker Creek Field, Arkansas: ABSTRACT

Porosity preservation in the Upper Smackover (Jurassic) carbonate grainstone, Walker Creek Field, Arkansas; response of paleophreatic lenses to burial processes; discussions and replies

Porosity preservation in the Upper Smackover (Jurassic) carbonate grainstone, Walker Creek Field, Arkansas; response of paleophreatic lenses to burial processes; discussions and replies

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: ADDITIONAL DISCUSSION

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: ADDITIONAL DISCUSSION

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: REPLY

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: REPLY

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: REPLY

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: REPLY

Porosity preservation in the upper Smackover (Jurassic) carbonate grainstone, Walker Creek Field, Arkansas; response of paleophreatic lenses to burial processes; reply

Porosity preservation in the upper Smackover (Jurassic) carbonate grainstone, Walker Creek Field, Arkansas; response of paleophreatic lenses to burial processes; reply

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes

A section of the ARCO Bodcaw No. 1 core from the Upper Smackover (Jurassic) of southern Arkansas was studied in detail in an effort to understand the control of porosity. Petrographic, isotopic, atomic absorption, and cathodoluminescence data on whole rocks and components were generated on 24 samples from a 115-foot length of core. The core is composed almost entirely of accretionary and cryptocrystalline grains and cement. Porosity is unrelated to grain type, size. or sorting on a sample-to-sample basis. The porous intervals are interpreted to have been stablized to calcite under fresh-water phreatic conditions. The porous intervals approach values of -5.0 per thousand delta 18 O, 1.5 mol percent Mg, and 300 ppm Sr and are further characterized by luminescent oolites and relatively slight solution compaction effects. The nonporous intervals represent relict vadose and subphreatic environments which stabilized after burial. The nonporous intervals approach values of -2.8 per thousand delta 18 O, 2.5 mol percent Mg, and 600 ppm Sr and are further characterized by nonluminescent grains and intense solution compaction. Data indicate that oolites were initially composed of a metastable mineralogy, cryptocrystalline grains were composed of a more stable mineralogy, and algal grains were composed of a mixture of mineralogies. Oolites in the vadose and subphreatic environments were not fully stabilized under fresh water conditions, but vadose grains carried with them a light delta 13 C soil gas signal after burial. These grains were fully stabilized under post-fresh-water, burial conditions. Observed porosity distribution is readily explained in terms of solution compaction and cementation. Grains with metastable mineralogy in relict vadose and subphreatic intervals were heavily solution-compacted at relatively shallow depths before being converted to stable calcite. Intense solution compaction destroyed pore space in these intervals. Dolomite cementation seems linked with the compaction process. The stablized mineralogy of the phreatic lens, together with associated meteoric cements, resisted solution compaction and thereby preserved pore space.

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: DISCUSSION

Porosity Preservation in the Upper Smackover (Jurassic) Carbonate Grainstone, Walker Creek Field, Arkansas: Response of Paleophreatic Lenses to Burial Processes: DISCUSSION

Walker Creek Revisited: Reinterpretation of Diagenesis of Smackover Formation, Walker Creek Field, Arkansas: ABSTRACT

End_Page 1675------------------------------Recent petrographic, trace element, isotopic, and fluid inclusion work has necessitated a major revision of the diagenetic model for the Smackover Formation at Walker Creek field, southern Arkansas. Prior studies concluded that all diagenetic changes occurred within a few tens of feet of the earth's surface. This investigation has shown that near surface diagenesis is confined to very minor marine cementation. The equant calcspar and baroque dolomite cementation occurred much later at temperatures between 60 and 120°C during the initial stages of hydrocarbon migration in the deep subsurface. Earlier workers also concluded that the equant cementation is unrelated to depositional facies. They attributed the layered permeable and impermeable zones to permeability preservation in a meteoric vadose zone and occlusion in a meteoric phreatic zone. Because the equant cement is of a late origin, it can no longer be attributed to a meteoric zone. The equant cement does occur in all depositional facies, but the effect of this cement upon the permeability of the rock is related to the original depositional facies. Permeability has been preserved in the coarser grained ooid and ooid-intraclast grainstones. This may be related to the size of the pore throats; cementation could block the pore throats completely in the finer grained material only. The coarser rock remained permeable and was made more permeable by late stage dissolution. This new-found relation of permeability and original depositional facies has several implications regarding future hydrocarbon exploration in the southern Arkansas-northern Lousiana area. End_of_Article - Last_Page 1676------------

Late Jurassic Reefs of Smackover Formation--Preliminary Report: ABSTRACT

Algal and coral reefs are recognized in conventional cores of the upper Smackover Formation from southwestern Arkansas eastward into the panhandle of Florida. Although only one known reef has produced commercial hydrocarbons, attractive porosities and permeabilities (mean ^phgr of 15%, mean K of 20 md) result from freshwater leaching, fracturing, or dolomitization. In addition, the reefs may have provided a positive structural aspect to localized areas during later Smackover deposition and diagenesis. Smackover reefs formed in the Late Jurassic during periods of maximum marine transgression (good circulation, clear water, normal marine salinity) in three major paleogeographic settings: (1) the margins of Paleozoic highs protruding into the Smackover basin, i.e., Vocation field in Alabama; (2) upthrown basement fault blocks, i.e., Melvin field in Alabama; and (3) the seaward edges of upthrown salt-cored fault blocks, i.e., Walker Creek field in Arkansas, Hico Knowles and North Haynesville fields in Louisiana, and West Paulding field in Mississippi. The buildups are commonly elongate, 3 to 40 m thick and generally cover an area of several square kilometers. The reefs appear higher (younger) in the stratigraphic section downdip. Also, the reefs are younger and have a more diverse biot in Arkansas and Louisiana than they do in Alabama and Florida. Smackover reefs in Alabama and Florida were constructed by algae. Vertical relief on the reef surface during growth may have been a few meters. Similar reefs in southern Arkansas and northern Louisiana exhibit a vertical zonation suggesting an evolving reef community. These buildups are Tubiphytes-stromatolitic algal boundstones containing scattered corals toward the base; diversity increases upward with the addition of abundant corals (Actinastrea, Complexastrea, Thamnasteria, and others), sponges, skeletal algae, and byrozoans. The reefs are commonly underlain and overlain by subtidal peloidal lime packstones containing oncolites and scattered fossils, and they can develop in close proximity to subtidal quartz sands. End_of_Article - Last_Page 1675------------

Seismic Stratigraphy in Low-Energy Carbonate Depositional Environment: ABSTRACT

Seismic stratigraphic mapping in carbonate rocks has been historically concentrated on the shelf margin environment where unit geometry and abrupt facies-velocity changes are often easily discernible with the seismic tool. In contrast, low-energy, more subtle shelf interior stratigraphic features have attracted less geophysical attention. This report describes the study of seismic data from five United States oil fields which produce from reservoirs formed from shelf interior deposits. Three Smackover fields, a Permian basin producer, and a Williston basin example are compared. Review of the depositional model indicates the difficulty of seismic mapping in non-reef carbonates. If sedimentary accretion is nearly vertical, porosity traps are predicted to exhibit only lateral change in unit velocity. In regressive or transgressive sequences, the geologic model indicates that angular discordance due to sediment buildup in combination with a velocity contrast generates an angular feature that has a different velocity pattern. These five fields provide examples of both trap types. Jay field (Florida) and Walker Creek field (Arkansas) both produce from the Smackover Formation and are (seismic-defined) stratigraphic traps which illustrate angular discordance and velocity contrast owing to regressive buildup. The trapping mechanism at Big Escambia Creek (Alabama), also productive from the Smackover, is from a transgressive pulse devoid of vertical buildup. Hence, the porosity trap is indicated only by a lateral velocity change without angular discordance. Subsurface geologic sections of San Andres production End_Page 798------------------------------ at Cato San Andres field (New Mexico) and Mission Canyon pay at Haas field (North Dakota) reveal no diachronous buildup. As expected these fields demonstrate only a lateral change in velocity within the formation. Application of stratigraphic-seismic techniques to low-energy carbonate facies can fall into two depositionally controlled divisions. When sediment accretes vertically, traps are mapped solely by lateral changes in velocity. Sedimentary buildup in transgressive or regressive sequences provides angularity, which combined with velocity contrast facilitates seismic detection of the trap. End_of_Article - Last_Page 799------------

Seismic-Stratigraphic Mapping of Gulf Coast Stratigraphic Traps: ABSTRACT

The mappability of a seismic-stratigraphic trap depends not only on the thickness and the stratigraphic position of the objective unit, but the velocity contrast with nearby beds. Analysis of band-pass filtered sonic logs provides a useful technique for determining the portion of the frequency spectrum that carries the basic stratigraphic information. Depending on local stratigraphy, both high (75 to 125 hz) and low (0 to 10 hz) frequency components may be important in defining the trap. The filtered sonic can be used to predict the seismic mappability of stratigraphic units. Review of seismic-stratigraphic data over fields in the Gulf Coast indicates that determining the mappability of a feature depends on complete understanding of the trap. At Walker Creek field, Smackover porosity is not resolved with a 60-hz filtered sonic. Because the basic reservoir-seal relation is low frequency, however, the field limits are clearly expressed on real seismic-stratigraphic data. Alternatively, recognition of the pinch-out of the 20-ft (6.1 m) thick Spanish Camp Sand at South Lissie field serves as an excellent example of a trap expressed as a high frequency feature. These simple examples illustrate clearly that both high and low frequency components are required for successful seismic mapping of Gulf Coast stratigraphic traps. End_of_Article - Last_Page 1568------------

Walker Creek Field, Arkansas--Smackover Case History: ABSTRACT

Walker Creek field is a stratigraphic trap containing 100 million bbl of oil and 100 Bcf of gas. The field lies End_Page 544------------------------------ 4 mi north of the Arkansas-Louisiana border in Columbia and Lafayette Counties, Arkansas. The trap is provided by the northern limit of a Smackover oolite facies which changes to equivalent Buckner shales and fine-grained limestones. The geologic model proposes that additional bar units, of slightly older geologic age, could be developed farther updip. A new 7-mi (11 km) seismic line was recorded north-south across the field. Two displays of conventionally processed seismic data define a subtle amplitude feature at the apparent Walker Creek field limits. One seismic display contains data with a 10-55-Hz filter, and a broader band section, containing 0-110-Hz data, certainly adds to the trap definition. Seislog (trademark of Teknika) inversion of these seismic data and detailed correlation of sonic logs to the Seislog line provide further definition of the facies geometry creating the amplitude feature. Definition of the updip limit of the 100 ft (30 m) of producing Smackover oolite is offered on the inverted seismic traces from 10, 800 ft (3,292 m; 2.1 sec). End_of_Article - Last_Page 545------------

Smackover Stratigraphic Traps--New Production in Old"Areas: ABSTRACT"

Stratigraphy has been recognized progressively as an important factor in the entrapment of Smackover hydrocarbons in the mature Arkansas and Louisiana producing fairway. Although most of the older fields generally were considered to be essentially in structural features, the importance of stratigraphic factors in entrapment has been dramatically focused by the discovery of Walker Creek field, Lafayette and Columbia Counties, Arkansas. The accumulation in this field is caused by stratigraphic entrapment by the updip termination of porous Smackover beds across a gentle structural nose. Discovery of Walker Creek End_Page 419------------------------------ signals the beginning of the second phase of Smackover exploration--the search for combination structural/stratigraphic and wholly stratigraphic traps--and the rebirth of exploration for large reserves in a mature segment of the trend. The regionally regressive depositional character of the Smackover in this area afforded an excellent setting for the formation of many stratigraphic traps. Porous carbonate zones, successively higher within the Smackover section, were deposited southward across the shelf. The updip terminus of each zone abuts an impermeable seal to form an ideal stratigraphic trap. The sinuous nature of the updip terminus, in many places in conjunction with low relief structural noses or closures, entraps the hydrocarbon accumulation laterally. In addition, many variations in the regional situation, due to the local depositional patterns of individual zones, tend to complicate the simple stratigraphic trap. Lithologically, the most characteristics reservoir rock is an oolitic-pelletal limestone with intergranular porosity. Porosity up to 30% is not unusual, but average porosity ranges from 10 to 20%. Various degrees of porosity destruction have resulted from the infilling of the primary porosity with sparry calcite cement. Where wave action was not sufficient to winnow out carbonate muds, no primary porosity was developed. The diverse nature of the stratigraphic traps opens up unlimited exploration opportunity on acreage once considered worthless because it was not located on closed structures. The stratigraphic phase of exploration now promises to be as profitable as was the structural phase in this old producing area. End_of_Article - Last_Page 420------------