North Louisiana Research Articles

Case History of Cotton Valley Sand Log Interpretation for a North Louisiana Field

Ganer, B.L., Union Texas Petroleum Corp. Member SPE-AIME Abstract Accurate porosity and water saturation determination of Cotton Valley sandstone is difficult since porosities below 12% are typically encountered. In this low porosity range the errors introduced in the process of obtaining log derived porosity and water saturation values process of obtaining log derived porosity and water saturation values can cause significant misinterpretation of productive potential. Such errors can be due to the presence of secondary matrix mineralization, parameters selected to calculate porosity and water saturation, and parameters selected to calculate porosity and water saturation, and calibration inaccuracies. In the case of Cotton Valley sandstone, which is predominantly a sand-shale sequence, the occurrence of secondary minerals can be significant. The Cotton Valley sands are a complex intermingling of various minerals including quartz, chert, pyrite, mica, illite, chlorite and an assortment of carbonates. Carbonates in isolated intervals can account for as much as 50% of the rock matrix. The heavy minerals, when not accounted for, can result in pessimistic log derived porosity and hence overlooked commercial potential. Additionally, calibration inaccuracies can result in either optimistic or pessimistic porosity analysis further confounding commercial evaluation. Variations in grain size coupled with the mineral composition complicate permeability determination. The variables mentioned above account for the difficulty in predicting reserves experienced in Cotton Valley sand exploration. Introduction This paper addresses the porosity analysis, using log measurements, of Cotton Valley sands in nineteen wells of the Terryville Field operated by Union Texas Petroleum. For a listing of the wells analyzed please refer to Table 1. Terryville Field is located in Lincoln Parish, North-central Louisiana (Figure 1). Cotton Valley sand in Terryville Field are primarily a sand-shale sequence found at depths between 8,500' to 11,000'. They are Upper Jurassic in age bounded above by the Knowles Limestone and below by the Bossier shale (Figure 2). The sands generally have low porosity, low permeability, and calcareous cementation. Due to the low porosity and low permeability of Cotton Valley sand, massive hydraulic fracture treatments are typically performed in completion work. The Terryville Field deep Cotton Valley sand potential was discovered in 1978. In the course of the past four years several problems have arisen involving evaluation of the sand and reserve estimation. Log analysis was not adequately consistent with subsequent production data for thirty-four (34) completions in the field. At the onset of the study the capabilities of the log measurements to evaluate the Cotton Valley sands were in doubt. Alignment of core analysis and log analysis was fair. Water saturation and porosity productive limits for the sand were uncertain. Concurrent with porosity productive limits for the sand were uncertain. Concurrent with the initiation of the study, newly acquired computer software, was set up to process lag interpretations. This allowed for more rapid sampling and modeling of log data to provide accurate evaluation of log calibrations and reservoir characteristics as indicated by log measurement. Sands of the Cotton Valley sand-shale series are identified as the C, D, Bodcaw, Vaughn, McCrary, McFearin-Davis, Bollinger, Schuler, Sexton and Taylor. A typical Cotton Valley log section with correlative sand tops is shown in Figure 3. Completion attempts have been made in most of the sands. The C and D sands appear to be better developed and have been produced in older wells. produced in older wells. The objectives of the study were:Recognize logging tool miscalibration and make correctionDetermine petrophysical parameters for the Cotton Valley sandsReinterpret the field using new petrophysical parametersDetermine productive limits for log derived porosity and watersaturation to evaluate the productive potential of the CottonValley sands.

North Louisiana. Volume One: To 1865. Essays on the Region and Its History.

North Louisiana. Volume One: To 1865. Essays on the Region and Its History.

Successful Field Applications of CO2-Foam Fracturing Fluids in the Arkansas-Louisiana-Texas Region

Summary Field experience in the Arkansas-Louisiana-Texas (ArkLa-Tex) area has demonstrated that CO2-foam systems can be used successfully in low-permeability oil and/or gas sands and carbonates at depths of 2,900 to 14,000 ft 1884 to 4267 m], reservoir temperatures of 120 to 370 deg F [48 to 188 deg ], and reservoir pressures of 1,000 to 13,200 psi [7 to 91 MPa]. Because of the high density of the psi [7 to 91 MPa]. Because of the high density of the water/CO2 mixture, CO2 foam can be used in deep, hot formations without prohibitive wellhead treating pressures. pressures. Introduction Hydraulic fracturing has been used to stimulate oil and gas producing wells since the late 1940's, and today it is considered a standard completion procedure on many wells that produce in the south Arkansas-north Louisiana-east Texas region. 1 Technology associated with fracture treatments has advanced significantly. particularly during the last 10 years. As the price of oil and gas has escalated, the application of many new fluids systems, proppants, etc. has become economically feasible, and proppants, etc. has become economically feasible, and many tight formations that formerly were bypassed as noncommercial are now producing significant reserve volumes. CO2 has been added to fracturing fluids in various producing areas of the U.S. for many years. 2 Usually, CO2 producing areas of the U.S. for many years. 2 Usually, CO2 up to 500 scf/bbl [89 std m 3 /M 3 j fluid was added to assist in the post-treatment cleanup operations. In the late 1970's, CO2 was used initially in Texas at concentrations of up to 50% of the total injected volumes because of the associated improved fluid-loss efficiency and reduced total load water requirements. Foam fracturing with 70% CO2 as the internal phase of the fracturing fluid was attempted in the Ark-La-Tex region for the first time in early 1982. The field results to date indicate that the system has distinct advantages over more conventional gelled-water and nitrogen-foam fracturing fluids. CO2-foam fracturing fluid (1) provides a gas drive to assist in removing (load) fluids after the proppant has been placed in the formation, (2) establishes permeability to placed in the formation, (2) establishes permeability to gas within the formation volume that has been saturated by load fluids, and (3) minimizes the actual water volume required to place a given volume of proppant in the formation. The CO2 reacts with water in the foamto form carbonic acid, so that the overall pH of the system is reduced (thus reducing the damaging effect of the fluids), andto lower the interfacial tension of the load fluids so that they can be recovered more rapidly and efficiently. Fracture Stimulation History The Ark-La-Tex tristate area is characterized by sand or carbonate intervals with alternating shale sections from the surface to the Smackover zone at 10,000 to 11,800 ft [3048 to 3597 m]. A typical penetration to Smackover depth (Fig. 1) encounters more than 40 different zones that have produced commercially within the Ark-La-Tex region, and the more than 500 fields demonstrate that hydrocarbon sources, trapping mechanisms (usually structural anticlimes and/or stratigraphy), and reservoir-quality rock are present throughout this area. (Commercial production was first developed in south Arkansas, north production was first developed in south Arkansas, north Louisiana and east Texas in 1921, 1922, and 1927, respectively; however, the significant increase in gas prices in the late 1970's greatly accelerated the economic prices in the late 1970's greatly accelerated the economic development of the tight Jurassic zones (Cotton Valley, Haynesville, and Smackover) in many areas that had been identified years earlier. Historically, low-permeability gas sands in the Ark-La-Tex region were successfully fracture treated with non-complexed, water-based treatment fluids to stimulate production. In recent years, most operators took advantage production. In recent years, most operators took advantage of the improved proppant transport and greater stability of complexed gel systems. If load recovery was considered a problem, CO2 often was commingled with complexed gels to aid cleanup. Recovery was enhanced further by use of 70-quality (70% gas by volume) nitrogen foams; however, increased wellhead treating pressures caused by hydrostatic head loss made treatment of deeper zones prohibitive. prohibitive. The Pettet Lime formation was stimulated primarily with acid treatments of various volumes, and acids often were gelled or foamed with nitrogen to increase formation penetration in fracture acidizing treatments. Fracture treatment with proppants was seldom used in the Pettet. With the introduction Of CO2 foams to the area, many qualities of the complexed gel systems (i.e., improved proppant transport and good stability) were combined with proppant transport and good stability) were combined with the improved load recovery of nitrogen foams. JPT P. 80

Oil and Gas Developments in Arkansas, North Louisiana, and East Texas in 1984

Oil and Gas Developments in Arkansas, North Louisiana, and East Texas in 1984

Early Evolution of Salt Structures in North Louisiana Salt Basin: ABSTRACT

Several salt diapirs and pillows in southern and central areas of north Louisiana have been studied using approximately 355 mi (570 km) of seismic reflection data and information from 57 deep well holes. Seismic profiles, with deep well-hole data, are the most advantageous method to document regional salt tectonism through time. Three stages--pillow, diapir, and postdiapir--are required to explain salt-stock growth through time in the North Louisiana salt basin. The pillow stage and its associated primary peripheral sinks exhibit 11-34% thinning over pillow crests and 12-50% overthickening in the primary peripheral sinks in the basin. Thinning values as great as 87% and overthickening values as high as 400% are inferred for prediapiric (juvenile) salt pillows. The diapir stage and its associated secondary peripheral sinks exhibit 50-250% overthickening. This stage is characterized by piercement diapirism and the withdrawal of large volumes of salt from the flanks of the pillow. The postdiapir stage and its associated peripheral sinks exhibit less than 45% overthickening. Moreover, in some instances dome growt is in a steady state with sedimentation. Growth stages are generally confined to the following stratigraphic units: Smackover to Terryville (pillow stage), Calvin (diapir stage), and Winn and younger sediments (postdiapir stage). These considerations lead to the following observations and conclusions on diapirism in the North Louisiana salt basin: (1) timing of the diapiric event commenced early (Late Jurassic) in the southern and central portion of the basin, and later (Early Cretaceous) in the northern portion; (2) the initial diapiric event is much more rapid and intense in the southern and central diapirs compared to the later diapiric event in the northern diapirs; and (3) regional depocenter shifting, relative sea level fall, local erosion with salt extrusion, and rapid depositional loading of sediments are the major controls on diapirism in the basin. End_of_Article - Last_Page 1426------------

Pedoturbation of a Forest Soil by Fire Ants

AbstractThe influences of the imported fire ant (Solenopsis invicta) on the mixing of forest soils on three sites in north Louisiana were examined. Soil bulk density, pH, percent organic matter, and concentrations of P, K, Ca, and Mg were compared for samples from ant mounds vs. adjacent undisturbed soil at two depths. In surface samples bulk density was lower and organic matter, P, K, Ca, and Mg were higher on the mounds. Soil pH did not change. In subsurface comparisons only organic matter and K differed and were higher in the mound samples. Results are discussed in terms of the depth of ant activity, the quantity of soil excavated, and the organic matter, P, K, Ca, and Mg content of that soil. Questions are raised regarding the role of fire ants in the decomposition of tree stumps.

Preliminary Paleomagnetic Investigations of Winnfield Salt Dome Cap Rock, Louisiana: ABSTRACT

Quarrying operations at Winnfield salt dome in the North Louisiana basin provide access to calcite, gypsum, and anhydrite cap-rock zones. Sulfide laminae in the anhydrite zone are comprised dominantly of pyrhotite with lesser amounts of sphalerite, galena, pyrite, and marcasite. Sulfides cement euhedral anhydrite grains and represent the products of the episodic introduction of metalliferous waters along the salt-anhydrite interface during halite dissolution and residual anhydrite accumulation. Thus, sulfide laminae provide a chronological record of anhydrite cap-rock accretion. Two hundred oriented samples were collected in stratigraphic sequence, covering much of the exposed anhydrite section. Alternating field demagnetization readily revealed the magnetic polarity of most samples. Using only reversely magnetized samples with a well-defined stable magnetization (N = 50, ^agr95 = 6.2°) yields a pole position at 71.4°N, 125.7°E, which implies that the sampled cap-rock sequence formed in the Late Jurassic. This age is consistent with geologic evidence indicating that cap-rock formation began in the late Jurassic and was most intense during the Early Cretaceous. A densely sampled 45-ft stratigraphic interval contains a sequence of normal and reverse polarity zones. Assuming a constant formation rate, these zones can be compared with the M-anomaly sequence. A growth rate of about 67 ft/m.y. (20 m/m.y.) is indicated. This value is about 30 times less than estimates of salt dome growth rates. Because a 10 to 50-fold decrease in volume is associated with halite dissolution and anhydrite accretion, the paleomagnetically determined value for cap-rock formation rate is reasonable. This investigation is the first such study undertaken. The results are encouraging and offer a unique approach for investigating the timing of various geologic processes related to salt dome formation. End_of_Article - Last_Page 1422------------

Thermal evolution of the north‐central Gulf Coast

The subsidence history of the North Louisiana Salt Basin, determined from well data, indicates that the region underwent extension during rifting and has since passively subsided due to conductive cooling of the lithosphere. Timing of the rifting event is consistent with opening of the Gulf of Mexico during Late Triassic to Early Jurassic time. Crustal extension by a factor of 1.5 to 2 was computed from “tectonic” subsidence curves. However, data from the early subsidence history are insufficient to distinguish between uniform and nonuniform extension of the lithosphere. The magnitude of extension is in good agreement with total sediment and crustal thicknesses from seismic refraction data in the adjacent Central Mississippi Salt Basin. The temperature distribution within the sediments is calculated using a simple heat conduction model. Temperature and subsidence effects of thermal insulation by overlying sediments are included. The computed temperature distribution is in good agreement with bottom hole temperatures measured in deep wells. Temperature histories predicted for selected stratigraphic horizons within the North Louisiana Salt Basin suggest that thermal conditions have been favorable for hydrocarbon generation in the older stata. Results from a two‐dimensional heat conduction model suggest that a probable cause for the early formation of the adjacent uplifts is lateral heat conduction from the basin. Rapid extension of the lithosphere underneath areas with horizontal dimensions of 50–100 km produces extremely rapid early subsidence due to lateral heat conduction. The moderate subsidence rate observed in the North Louisiana Salt Basin during the Jurassic and Early Cretaceous suggests slow extension over a long period of time.

The cut and recorrelated gravity map of the United States

Cover. (a) Filtered gravity map of the 48 contiguous states. Produced by Hildenbrand, Simpson, Godsen, and Kane as USGS Map GP‐953(A). (b) The map cut and recorrelated. Gravity signatures of the following major features are aligned. (1) Central North American Rift System, Hartville Uplift; (2) Precambrian/Early Paleozoic Rift Passive Margin underneath the Appalachians, Precambrian/Early Paleozoic Rift Passive Margin underneath the Quachitas. Though this offset was independently recognized by the author, it is similar to that proposed by S. W. Carey (The Expanding Earth, Dev. in Geotonics, vol. 10, pp. 378–380, Elsevier, New York, 1976). These alignments are interpreted to indicate that a strike‐slip fault system (transform fault?) offset these features probably concurrent with rift passive margin formation leaving the Palezoic Iapetian Eastern‐Southern continental margin bounded as follows: rift passive margin—transform passive margin—rift passive margin. Late Palezoic continental collision resulted in major overthrusting of the margin forming the Appalachians; lesser overthrusting, because of relatively recessed position, forming the Quachita's and minor thrusting with considerable shearing, because of the orientation of the transform boundary with respect to the closing vector, forming the buried Appalachian‐Quachita connection in Mississippi and Alabama (G. L. Kinsland, Transcontinental strike‐slip fault across North America, paper presented at the 5th International Conference on Basement Tectonics, Oct. 1983). Features south and east of the rift passive margins were not in place at the time faulting occurred and would not a priori be expected to correlate in (b). The small features which do correlate in (b) from north Louisiana northeast along the Atlantic coast are believed to be signatures of portions of collapsed Late Palezoic island arc complexes which correlate due to similar structural relationships to the rift passive margins. Florida does not correlate with southern Mississippi and Louisiana. At this time, no reason for extending the cut through Florida other than necessity to illustrate the other correlations is given. A signature similar to the rift passive margin signatures is noted in southern Mississippi, southern Louisiana, and coastal Texas and is interpreted as the Mesozoic Gulf Coast rift passive margin signature (G. L. Kinsland, Basement Structure of the Gulf Coast: Interpretation of gravity anmalies supported with structural, magnetic and seismic data, Trans. Gulf Coast Assoc. Geol. Soc, in press, 1984).

A strontium isotopic study of Smackover brines and associated solids, southern Arkansas

The isotopic composition of Sr has been measured in brine samples from the Upper Jurassic Smackover Formation in southern Arkansas; 87Sr 86Sr ratios range from 0.7071 to 0.7101. With one exception, the 32 Smackover brines contain Sr which is significantly more radiogenic than the Sr in Late Jurassic sea water, indicating sizable Sr contributions from detrital sources. Isotopic analyses of core samples from rock units associated with the brines and regional stratigraphic relationships suggest that the radiogenic Sr was released from detrital minerals in Bossier shale to interstitial fluids expelled from the underlying Louann Salt in the North Louisiana salt basin. These fluids migrated through the Bossier Formation updip to the South Arkansas shelf, where they entered the upper Smackover carbonate grainstone. The radiogenic fluids mixed with Sr-rich interstitial marine waters that had the isotopic composition of Late Jurassic sea water; mixing in variable proportions resulted in the random distribution pattern of variable 87Sr 86Sr ratios that is observed in Smackover brines within the 5000 km 2 study area. Isotopic analyses of nonskeletal carbonate grains and coexisting coarse calcspar cement from the upper Smackover grainstone imply that the grains were diagenetically stabilized in the presence of interstitial marine waters, whereas the calcspar cement is a relatively late diagenetic phase precipitated after the arrival of radiogenic fluids.

Gibsland Salt-Stock Family in Northwestern Louisiana: ABSTRACT

A semiregional isopach map of the Hosston-Sligo interval in north Louisiana suggests the existence of a salt-stock family similar to D. Sanneman's example in the Zechstein basin of northwestern Germany. The mother salt appears to be the Gibsland salt dome in Bienville Parish, which the isopach map indicated had a well-developed rim syncline during Hosston deposition. Withdrawal of salt into the Gibsland dome appears to have triggered the growth of peripheral salt pillows such as Vacherie, Minden, Athens, Sugar Creek, and Arcadia. Some of these pillows subsequently developed into salt stocks. The centrifugal or outward growth of salt structures continued with the withdrawal of salt from beneath the Minden subbasin into the Minden and Bistineau salt domes. This acce tuated growth of the Sligo, Bellevue, and Cotton Valley salt pillows, which in turn triggered development of the Pine Island salt pillow in latest Early Cretaceous time. The growth of the salt structures progressed outward from deeper to shallower portions of the North Louisiana salt basin. An older salt-stock family may be centered on the Winnfield or Cedar Creek salt domes in the deepest part of the salt basin. Centrifugal growth of these stock should be discernible in seismic profiles. A knowledge of the relative ages of these structures is important in predicting sites of Lower Cretaceous reefs and hydrocarbon migration paths. End_of_Article - Last_Page 1219------------

Oil and Gas Developments in Arkansas, North Louisiana, and East Texas in 1983

Oil and Gas Developments in Arkansas, North Louisiana, and East Texas in 1983

Metallic Sulfide Deposits in Winnfield Salt Dome, Louisiana: Evidence for Episodic Introduction of Metalliferous Brines During Cap Rock Formation: ABSTRACT

ABSTRACT Winnfield Dome is a shallow piercement salt structure which penetrates late Jurassic through early Tertiary siliciclastic and carbonate strata of the North Louisiana Basin. Quarrying operations in the calcite and anhydrite portions of the cap rock have exposed zones of metallic sulfides and barite. Within the anhydrite portion of the cap rock, sulfides form laminar zones parallel to the tightly intergrown, layered anhydrite; sulfides occur either as cement between anhydrite grains or as coarse euhedral crystals in open space. The sulfide layers are up to several centimeters thick and extend for tens of meters. These layers locally thicken into laminated, flat-bottomed concave downward massive sulfide mounds. Monoclinic pyrrhotite is the dominant sulfide; sphalerite, galena, or barite may also be present. In weathered samples hydrating to gypsum, marcasite is common and appears to be an alteration product of pyrrhotite. A roughly laminated massive sulfide lens is exposed at the calcite/anhydrite transition zone. This sulfide body has a maximum thickness of about 5 m (16 ft) and extends for about 45 m (150 ft). The edges of the lens taper to a layer about 30 cm (1 ft) thick which separates the overlying calcite from the anhydrite. The massive sulfide lens is comprised of marcasite and pyrite complexly intergrown with calcite, gypsum, and anhydrite. Colloform sphalerite, galena, marcasite, pyrite, and barite line vugs and fractures; pyrrhotite is found locally within the sulfide lens. Examination of rotary drill samples suggests that massive sulfide concentrations, similar to the exposed example, are common along the calcite/anhydrite transition zone. These sulfide concentrations are believed to have originated from the interaction of metalliferous basinal brines with reduced sulfur trapped within the cap rock. Textural relationships and variations in chemical compositions between the sulfide layers in the anhydrite portion of the cap rock suggest that distinct pulses of metalliferous brines were responsible for the sulfide concentrations. Anhydrite grains that are completely surrounded by sulfides are euhedral and undeformed, similar to the anhydrite disseminated throughout the salt mass. Anhydrite grains outside the mineralized areas are deformed and tightly intergrown. These textures suggest that mineralizing fluids were introduced episodically along the salt/anhydrite interface at the zone of salt dissolution, before that portion of the anhydrite zone was compressed and accreted to overlying anhydrite cap rock. Therefore, the earliest-formed sulfides originating by this mechanism occur at the top of the anhydrite cap rock zone while the last sulfides to form are found at the base. Extensive sulfide concentrations along the anhydrite/calcite contact suggest that this contact also acted as a permeable zone allowing metalliferous brines into the cap rock. Textural and compositional relationships suggest that sulfides that formed along the anhydrite/calcite contact are locally superimposed on sulfides that formed at the salt/anhydrite contact.

An Equation for Predicting Harvesting Costs on Second-Growth Southern Yellow Pine Sites in the Midsouth

Abstract Time-and-motion studies were conducted on four contract logging operations in north Louisiana to develop input parameters for the felling, skidding, loading, and transportation simulator (FSLAT). FSLAT was then used to simulate the harvest of 16 stands of various haul distances for four different contractors. A data base of 320 observations was constructed from the projected logging costs, stand variables, and haul distances. A step-wise variable selection package was used to select the best combination of variables for an equation to predict logging costs. The best equation, based on 1982 equipment and fuel costs, exclusive of profit, is: [equation] A fifth operation was studied for verification purposes. The equation predicted within 0.3 percent of the actual cost.

Mucocutaneous lymph node syndrome in North Louisiana: review of 15 cases.

Mucocutaneous lymph node syndrome (MLNS) is being more frequently reported in the United States, and it appears to be an important disease of childhood. Fifteen infants and children with MLNS were treated at the Louisiana State University teaching hospitals in Shreveport and Monroe between August 1978 and January 1981. A cluster of nine of the cases (60%) occurred between February and May 1980. In this series the male to female ratio was 2:1, and 53% of the patients were between 11 and 15 months of age. In five of the patients, platelet counts and erythrocyte sedimentation rates were monitored for at least four weeks.

Oil and Gas Developments in Arkansas, North Louisiana, and East Texas in 1982

The 1984 level of drilling activity for Arkansas, north Louisiana, and east Texas (CSD Districts 1, 9, and 24, respectively) varied slightly from 1983 levels. Both east Texas and north Louisiana had slight increases in the level of exploratory drilling, whereas Arkansas exploratory drilling decreased. However, success rates for exploratory drilling rose in all 3 areas. Development drilling decreased in east Texas and Arkansas, but increased substantially in north Louisiana. Success rates for development drilling increased in north Louisiana, but decreased in east Texas and Arkansas.

Subsidence and Heat Flow Across a Sedimentary Basin-Uplift Boundary: A Thermal-Mechanical Model: ABSTRACT

The northern coast of the Gulf of Mexico contains major depositional basins in east Texas, north Louisiana, and central Mississippi. They are separated by the Sabine and Monroe uplifts. Basin subsidence can be interpreted as resulting from extension by rifting related to opening of the Gulf of Mexico during Late Triassic to Early Jurassic times. Subsidence of the North Louisiana salt basin, determined from well data, is consistent with crustal extension by a factor of 1.5 to 2. Seismic surveys across End_Page 526------------------------------ the East Texas and Central Mississippi salt basins also indicate thinned crust. Thus, a probable explanation for the intervening Sabine and Monroe uplifts is that they represent areas of unstretched or only slightly stretched lithosphere. This juxtaposition of thermally perturbed lithosphere with unperturbed or only slightly perturbed material is similar to interactions between oceanic crust of different ages across a fracture zone. A thermal-mechanical model describing lateral conduction of heat and mechanical coupling of the lithosphere across this boundary is presented. Effects of finite extension rate, increasing mechanical thickness with age as the lithosphere cools, and thermal blanketing by overlying sediment are included. Subsidence history, heat flow, and sedimentary stratigr phy predicted from this model are compared with observational data. End_of_Article - Last_Page 527------------

Regional Patterns of Diagenesis, Porosity Evolution, and Hydrocarbon Production, Upper Smackover of Gulf Rim: ABSTRACT

The exploration fairway of the Upper Jurassic Smackover, from the Rio Grande to the Panhandle of Florida, consists of a rather simple carbonate ramp depositional system characterized by thick, widespread blanket ooid sands. The ooid sand belt gives way landward to quartzose End_Page 1468------------------------------ clastics, and locally to shales and evaporites. The high-energy ooid sand belt changes sharply into dark, fine-grained limestones and shales in a seaward direction. Early salt movement, buried basement structures, and growth faulting locally affected sedimentation patterns within the upper Smackover. Salt anticlines active during Smackover deposition leading to localization of favorable facies are particularly important along the east Texas and south Arkansas parts of the trend. Buried basement structures as well as salt structures controlled detailed sedimentation patterns in the Alabama-Florida parts of the trend, while growth faults controlled sedimentation along a narrow belt straddling the Arkansas-Louisiana border. Initial porosities and permeabilities were, however, generally h gh across the entire Smackover fairway prior to burial. Presently observed porosity-permeability trends are controlled generally by postdepositional processes including compaction, dissolution, cementation, and dolomitization. Regional differences in burial history along the Gulf rim, related in part to proximity to the isolated interior salt basins (Mississippi, north Louisiana, and east Texas salt basins), have resulted in striking differences in reservoir characteristics across the trend, reflecting significant regional differences in pathways of porosity evolution. East Texas is a province dominated by dolomitized reservoirs, with production controlled by proximity to major fault trends, such as the Mexia-Talco, or to well-defined salt anticlines. Reservoir quality is tied inextricably to dolomitization. Dolomitization occurred early, associated with a regional fresh meteoric water system, hence reservoir characteristics were generally established prior to significant burial. Fracturing is a general feature of east Texas sequences, and there is evidence that fractures have acted as conduits for hydrocarbon migration. East Texas is generally a gas province, with oil migration occurring relatively early in the burial history of the sequence. Late burial diagenetic events in east Texas seem to have had little influence over ultimate porosity evolut on. Because of the depths involved, the south Texas Jurassic has been explored only superficially and no production has been established. Recent works by Loucks and Budd on the available material indicates strong similarity to east Texas relative to porosity evolution and general diagenetic framework. However, Loucks and Budd do note significant, late, subsurface, secondary porosity development. South Arkansas, Louisiana, and Mississippi stand out in stark contrast to the Texas part of the trend. Reservoirs are generally limestones, with porosity either early fresh meteoric, secondary moldic, or preserved primary porosity. The early meteoric moldic porosity occurs in a predictable trend across the updip portion of the fairway. Reservoirs with preserved primary porosity occur in a band seaward of the secondary trend, and show no evidence of freshwater influence, or early diagenetic processes other than minor marine cements. Porosity preservation in this zone is a function of grain type (ooids versus pellets) and original sediment texture, and hence it is controlled ultimately by depositional processes. In the primary porosity zone, production is almost always associated with s lt-related structures, while in the early secondary zone, updip permeability barriers (diagenetic and stratigraphic?) as well as salt-related structures are important. Late diagenetic events associated with migration of basinal derived fluids across the shelf during moderate burial include cementation dedolomitization, and calcite dissolution. All reservoirs in this part of the trend show ample evidence of significant porosity enhancement during this late solution phase. The limestone trend of Arkansas, Louisiana, and Mississippi is primarily an oil province with hydrocarbon migration taking place much later than to the west in Texas. In Alabama and Florida, the trend is again toward dolomite reservoirs, with most dolomite and hence porosity permeability establishment early, and associated with meteoric water processes. Most large reservoirs such as Jay are associated with salt anticlines; minor production comes from updip basement structures. This part of the trend is mixed gas and oil. Hydrocarbon migration into reservoirs seems to have been a relatively late event. The Jurassic upper Smackover of the Gulf rim is a simple sedimentologic system that has seen a complex and variable burial history along the trend which is distinctly reflected in major changes in diagenetic history, reservoir-porosity type, trap characteristics, and hydrocarbon migration timing. End_of_Article - Last_Page 1469------------

Exploration and Development--Gulf Coast Mesozoic, 1982 and 1983: ABSTRACT

Looking at exploration and development within the Gulf Coast Mesozoic region as a whole, the trend has been toward shallow oil and away from deep gas. The capability of each area to successfully accommodate this shift is dramatically reflected in the comparison of total wells drilled in 1982 as compared to 1981: Table The southeastern states were hard hit because this area was so strongly oriented toward deep Hosston and Cotton Valley gas plays, and there were no viable shallow oil plays available as quick alternatives. South Arkansas and north Louisiana suffered some from the downturn in Smackover and Cotton Valley exploration (note total wells much greater than in the southeastern states), but there were numerous plays to be made in the Upper Cretaceous Sands, Wash.-Fred., Paluxy, Rodessa, Sligo, and Hosston. East Texas actually faired better in 1982 than 1981, in part because of the many shallow oil plays to be made, but also because, according to G.C.S. statistics, the shift to shallow oil had actually begun between 1980 and 1981, so momentum in this direction was already present. South Texas s w the demise of the Austin Chalk play within the Giddings trend through Lee and Fayette Counties. Nevertheless, the Chalk play remains active in Atascosa and Wilson Counties. The most significant trends in the southeastern states during 1982 were the Rodessa-Cotton Valley play centered in Warren and Hinds Counties, Mississippi, and the Miocene play of Baldwin and Mobile Counties, Alabama. The most significant discovery was the Movico Smackover field on the west flank of the Mobile graben in Mobile County, Alabama. In 1983, an apparent major Smackover discovery has been made by Beau Coupe Oil & Gas in extreme southeastern Escambia County, Alabama. Also, there is an important play developing in the lower Tuscaloosa on the shelf slope between the low relief anticlines of northwestern Mississippi and the Tuscaloosa growth faults of southern Louisiana. Although exploration in north Louisiana and south Arkansas was relatively strong, most activity was concentrated in or around existing fields. The west and north flanks of the Pine Island dome located in Caddo Parish, Louisiana, were the sites of very intense activity. Exploration and development are directed toward the Sligo with secondary pays in the Rodessa and Hosston. In all, 44 wells have been drilled to these pays in and around the shallow Pine Island Pettit oil field. East Texas activity was dominated by discoveries and development of Rodessa and Pettit fields in the basin and on the west and south flanks of the Sabine uplift. However, the most significant discovery last year was the East Ginger Smackover field in Rains County. In early 1983, Cities Service made headlines with what appeared to be a major discovery in eastern Cass County. A 40-ft (12-m) pre-Smackover sand made a strong flow of oil and gas before going to water. As 1982 came to a close, it appeared that south Texas would have an important play in the Sligo, seaward of the Stuart City Reef trend. However, at this time, commercial production from the Sligo has not been established definitely. End_of_Article - Last_Page 1474------------

Application of Strontium Isotopes to Origin of Smackover Brines and Diagenetic Phases, Southern Arkansas: ABSTRACT

The abundance of the isotope 87Sr is variable in nature, as it is the radiogenic product of 87Rb decay. The relative amount of this Sr isotope that is dissolved in a brine, as expressed by the 87Sr/86Sr ratio, might be used as a tracer of the origin and subsequent history of the brine, including its diagenetic effects in petroleum reservoirs. Strontium isotopic analyses of 40 brines from oil fields in southern Arkansas have been conducted to investigate the sources of the dissolved Sr, the pathways of brine migration, and the relationship between the brines and diagenetic phases in the Jurassic upper Smackover Formation. The 87Sr/86Sr ratios of 33 brines from the upper Smackover lime-grainstone range from 0 7071 to 0.7101; seven brines from formations stratigraphically above the Smackover range from 0.7090 to 0.7112. Thus the Sr in these brines is variably more radiogenic than Jurassic sea water End_Page 553------------------------------ (~0.7070). The brines are from 15 oil fields within an area of approximately 2,000 mi2 (5,200 km2); the isotopic variability shows no geographic pattern in the study area. The variation in 87Sr/86Sr ratios of brines occurs to a lesser degree within individual oil fields. For example, over a distance of about 10 mi (16 km) at Walker Creek, the isotopic ratios of 11 Smackover brines range from 0.7080 in the east to 0.7086 in the west. The observation that Smackover brines are variably more radiogenic than Jurassic seawater is important because it indicates that a significant proportion of the Sr dissolved in these brines has been acquired from a source material that has not formed wholly by precipitation from Jurassic seawater. That is, some radiogenic Sr must have been added to the brines from a detrital source material. The nature and distribution pattern of the 87Sr/86Sr ratios indicate the acquisition of variable amounts of radiogenic Sr on a local basis. If the Smackover brines originated in the Louann Salt, with 87Sr/86Sr equivalent to that of Jurassic seawater, their present isotopic compositions may be the result of varying degrees of subsequent interaction with d trital sediments or they may have been produced by mixing in variable proportions with solutions containing more radiogenic Sr. Potential sources of the radiogenic Sr are the Norphlet Formation and the lower Smackover argillaceous lime-mudstone, both of which lie stratigraphically between the Louann salt and the upper Smackover, as well as the Bossier shale which interfingers with the upper Smackover in the North Louisiana salt basin. Anhydrites from the Werner and Buckner formations and from northern Louisiana salt domes, which constitute additional potential sources of brine Sr, yield 87Sr/86Sr ratios equivalent to those of Jurassic seawater. Diagenetic phases of the upper Smackover, such as post compaction calcspar cement and baroque dolomite, have 87Sr/86Sr ratios more radiogenic than Jurassic seawater, suggesting their subsurface precipitation in Sr isotopic equilibrium with Smackover brines. However, ooids and oncolites from the upper Smackover lime-grainstone yield 87Sr/86Sr ratios indicating isotopic equilibrium with Jurassic seawater. End_of_Article - Last_Page 554------------