Marble Falls Limestone Research Articles

Volumetric aberrancy to map subtle faults and flexures

One of the key tasks of a seismic interpreter is to map lateral changes in surfaces, not only including faults, folds, and flexures, but also incisements, diapirism, and dissolution features. Volumetrically, coherence provides rapid visualization of faults and curvature provides rapid visualization of folds and flexures. Aberrancy measures the lateral change (or gradient) of curvature along a picked or inferred surface. Aberrancy complements curvature and coherence. In normally faulted terrains, the aberrancy anomaly will track the coherence anomaly and fall between the most positive curvature anomaly defining the footwall and the most negative curvature anomaly defining the hanging wall. Aberrancy can delineate faults whose throw falls below the seismic resolution or is distributed across a suite of smaller conjugate faults that do not exhibit a coherence anomaly. Previously limited to horizon computations, we extend aberrancy to uninterpreted seismic data volumes. We apply our volumetric aberrancy calculation to a data volume acquired over the Barnett Shale gas reservoir of the Fort Worth Basin, Texas. In this area, the Barnett Shale is bound on the top by the Marble Falls Limestone and on the bottom by the Ellenburger Dolomite. Basement faulting controls karstification in the Ellenburger, resulting in the well-known “string of pearls” pattern seen on coherence images. Aberrancy delineates small karst features, which are, in many places, too smoothly varying to be detected by coherence. Equally important, aberrancy provides the azimuthal orientation of the fault and flexure anomalies.

Correlation analysis of fracture arrangement in space

We present new techniques that overcome limitations of standard approaches to documenting spatial arrangement. The new techniques directly quantify spatial arrangement by normalizing to expected values for randomly arranged fractures. The techniques differ in terms of computational intensity, robustness of results, ability to detect anti-correlation, and use of fracture size data. Variation of spatial arrangement across a broad range of length scales facilitates distinguishing clustered and periodic arrangements—opposite forms of organization—from random arrangements. Moreover, self-organized arrangements can be distinguished from arrangements due to extrinsic organization. Traditional techniques for analysis of fracture spacing are hamstrung because they account neither for the sequence of fracture spacings nor for possible coordination between fracture size and position, attributes accounted for by our methods.All of the new techniques reveal fractal clustering in a test case of veins, or cement-filled opening-mode fractures, in Pennsylvanian Marble Falls Limestone. The observed arrangement is readily distinguishable from random and periodic arrangements. Comparison of results that account for fracture size with results that ignore fracture size demonstrates that spatial arrangement is dominated by the sequence of fracture spacings, rather than coordination of fracture size with position. Fracture size and position are not completely independent in this example, however, because large fractures are more clustered than small fractures. Both spatial and size organization of veins here probably emerged from fracture interaction during growth. The new approaches described here, along with freely available software to implement the techniques, can be applied with effect to a wide range of structures, or indeed many other phenomena such as drilling response, where spatial heterogeneity is an issue.

Hydraulic Fracturing: Compensation of Surge and Swab Pressures in Floating Drilling Operations

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 110562, "Contacting More of the Barnett Shale Through an Integration of Real-Time-Microseismic Monitoring, Petrophysics, and Hydraulic-Fracture Design," by J. Daniels, SPE, G. Waters, SPE, J. LeCalvez, SPE, J. Lassek, SPE, and D. Bentley, Schlumberger, prepared for the 2007 SPE Annual Technical Conference and Exhibition, Anaheim, California, 11-14 November. The paper has not been peer reviewed. With effective stimulation techniques, economics for horizontal wells in shale-gas reservoirs are favorable compared to vertical wells. Real-time fracture mapping enables on-the-fly changes in fracture design to maximize the effective stimulation volume (ESV). ESV is defined as the reservoir volume effectively contacted by the stimulation treatment as determined by microseismic (MS) event locations and density. Correlation of MS activity with log data enables estimating fracture geometry. These data then are used to design a fit-for-purpose stimulation that has the greatest chance of maximizing the ESV and production. Introduction The Barnett shale is a Mississippian marine-shelf deposit that lies unconformably on the Ordovician Viola limestone/Ellenberger group and is conformably overlain by the Pennsylvanian Marble Falls limestone. The Barnett shale, within the Fort Worth basin, was the focus of this study, concentrating on wells in Denton, Wise, and Tarrant counties of Texas: the "core area." The Barnett in the core area ranges from 300 to 500 ft thick. Matrix permeabilities range from 0.00007 to 0.0005 md, with porosities ranging from 3 to 5%. The Barnett is its own source rock and is abnormally pressured in this area, with a pore-pressure gradient of approximately 0.5 psi/ft. Hydraulic-fracture stimulation is required to obtain commercial production. Before 1997, Barnett wells were completed with massive hydraulic-fracture treatments of crosslinked gelled fluids and large amounts of proppant. Difficulties in cleaning up fracture damage from the crosslinked gel and the high cost of these massive stimulation treatments led to marginally economical wells. In 1997, large-volume high-rate slick-water fracture-stimulation treatments were tested. While well performance was not increased significantly, completion costs were reduced by approximately 65%. In 2002, horizontal wells were drilled to increase wellbore exposure to the reservoir. The first horizontal wells increased the estimated ultimate recovery by 300% at double the well cost relative to vertical wells. Horizontal wells offer an economic solution outside the core area because fracture-height growth can be reduced while creating a large fracture-surface area. This solution is critical for regions in which the shale is bounded by potentially water-producing formations. Horizontal wells reduce the number of surface locations needed near populated areas and have been used in various infill-drilling programs. Gas-recovery factors can be improved by focusing the stimulation treatments on zones in the lateral section that have not been contacted by offset wells.

Total petroleum system assessment of undiscovered resources in the giant Barnett Shale continuous (unconventional) gas accumulation, Fort Worth Basin, Texas

Undiscovered natural gas having potential for additions to reserves in the Mississippian Barnett Shale of the Fort Worth Basin, north-central Texas, was assessed using the total petroleum system assessment unit concept and a cell-based methodology for continuous-type (unconventional) resources. The Barnett-Paleozoic total petroleum system is defined in the Bend arch–Fort Worth Basin as encompassing the area in which the organic-rich Barnett is the primary source rock for oil and gas produced from Paleozoic carbonate and clastic reservoirs. Exploration, technology, and drilling in the Barnett Shale play have rapidly evolved in recent years, with about 3500 vertical and 1000 horizontal wells completed in the Barnett through 2005 and more than 85% of the them completed since 1999. Using framework geology and historical production data, assessment of the Barnett Shale was performed by the U.S. Geological Survey using vertical wells at the peak of vertical well completions and before a transition to completions with horizontal wells. The assessment was performed after (1) mapping critical geological and geochemical parameters to define assessment unit areas with future potential, (2) defining distributions of drainage area (cell size) and estimating ultimate recovery per cell, and (3) estimating future success rates. Two assessment units are defined and assessed for the Barnett Shale continuous gas accumulation, resulting in a total mean undiscovered volume having potential for additions to reserves of 26.2 TCFG. The greater Newark East fracture-barrier continuous Barnett Shale gas assessment unit represents a core-producing area where thick, organic-rich, siliceous Barnett Shale is within the thermal window for gas generation (Ro 1.1%) and is overlain and underlain by impermeable limestone barriers (Pennsylvanian Marble Falls Limestone and Ordovician Viola Limestone, respectively) that serve to confine induced fractures during well completion to maximize gas recovery. The extended continuous Barnett Shale gas assessment unit, which had been less explored, defines a geographic area where Barnett Shale is (1) within the thermal window for gas generation, (2) greater than 100 ft (30 m) thick, and (3) where at least one impermeable limestone barrier is absent. Mean undiscovered gas having potential for additions to reserves in the greater Newark East assessment unit is estimated at 14.6 tcf, and in the less tested extended assessment unit, a mean resource is estimated at 11.6 TCFG. A third hypothetical basin-arch Barnett Shale oil assessment unit was defined but not assessed because of a lack of production data.

Middle Pennsylvanian Strike-Slip Faulting on the Eastern Edge of the Llano Uplift, Central Texas

ABSTRACT Pre-Strawn rocks of the Llano Uplift, Central Texas, are cut by a pervasive system of NE-trending faults whose movement more-or-less coincides with the culmination of Ouachita orogeny in the area. Map patterns indicate that major faults are near-vertical, which can be explained best by strike-slip faulting. Right-lateral displacements probably dominate near Marble Falls, Texas. The complex fault slice centered on Marble Falls, near the eastern edge of the Paleozoic outcrop, is more than 30 km long but only 2-8 km wide. Structure within this block exhibits a syncline and an anticline having axes at about N60W that is, dips are NE in the southwestern part of the block, SW for 5 or 6 km NE of Marble Falls, and thence NE toward the unconformable Cretaceous cover. The eastern bounding fault at Mormon Mills has an apparent vertical displacement of 500 meters or so (Marble Falls Limestone and Smithwick Shale against Tanyard Dolomite). The western boundary fault has an apparent vertical displacement of more than 1 km at Marble Falls (Smithwick Shale and Marble Falls Limestone against Town Mountain Granite) but to the N splinters into several left-stepping blocks. Some splinter faults die out northward within a few hundred meters to zero displacement, and the western bounding fault of the dominant splinter block some 10 km N of Marble Falls has only 100-200 meters apparent vertical displacement (Tanyard Dolomite against San Saba Dolomite). Much of the differential displacement clearly was accommodated by folding of strata within the fault block, although cross-faults are abundant. Outcrop and quarry exposures E of U. S. 281, near Sudduth Siding, show that the Threadgill Member of the Tanyard Formation and the overlying Staendebach Dolomite, near the crest of the broad antiform, are shattered by fractures in sets that trend N20W, N80E, N40-50E, and N30-40W. The N20W and N80E sets are most prominent on 1940 and current aerial photographs; these sets bound the Brownlee Ranch Dolomite Quarry (Dow Chemical USA) on the west and north. Randomly-oriented blocks of chert-bearing Staendebach Dolomite were dropped into breccia zones along NE-SW and E-W fracture sets within Threadgill Dolomite. No younger material was structurally incorporated into the breccia. Where fractures are well-exposed in solid rock between breccia zones, little vertical displacement could be discerned across most individual fractures, but nearly-horizontal to shallowly-dipping slickensides and mullions are common. Some fractures of the N20W, N40-50W, and N40-50E sets display "flower structures" within the quarry on a scale that cuts dolomite into vertically-bounded blocks. In pasture outcrops these features can be mistaken easily for vertically-dipping beds 5 cm to 1 m thick. We believe that the N20W/N80E and the N40-50E/N40-50W fracture sets are pairs that formed at different times under different stress conditions, but could not determine unequivocal temporal relationships. Saddle dolomite druse in single to multiple layers coats fracture faces of all sets and no vertical or steeply-dipping stylolites were found, although horizontal stylolites are abundant. The gross morphology of the Ouachita belt indicates first-order westward movement of the orogen in Central Texas. We suggest that as the orogen impacted the generally NW-SE trend of the Central Texas craton, extensive strike-slip faults sliced the craton into blocks, producing gentle folds within blocks during the process. Blocks also were extensively fractured internally, probably during more than one horizontally-compressive stress event. Along the eastern exposed margin of the Central Texas craton, horizontal compression ceased before fractures could be closed and modified by pressure-solution into vertical stylolites. End_of_Record - Last_Page 11--------

Fusulinacean biostratigraphy of the Marble Falls Limestone (Pennsylvanian), western Llano region, central Texas

Fusulinacean biostratigraphy of the Marble Falls Limestone (Pennsylvanian), western Llano region, central Texas

Geochemical Signatures in Carbonate Matrix and their Relation to Deposition and Diagenesis, Pennsylvanian Marble Falls Limestone, Central Texas

ABSTRACT Stable isotope ratios, strontium 87/86 ratios, and magnesium, strontium, and iron contents have been measured on carbonate matrix and on bulk-rock biomicrites in Early Pennsylvanian limestone (Marble Falls Formation) in central Texas. Patterns have been found which relate to primary conditions in the depositional environment, such as salinity and mineralogy, and to early diagenesis. Oxygen isotopes disclose differences of salinity across the ancient Llano Platform that extended from the Fort Worth Basin (east) to the Concho Arch (west). Open-marine circulation which existed basinward of the platform involved seawater with a possible 18O of -1.5 per mil (SMOW), whereas 18O enrichment in red biomicrites behind platform margin shoals reveal poor circulation and evaporation of seawater to a 18O of at least +1 (SMOW) in that environment. The platform margin shoals are composed of oosparites and biosparites with moderately low bulk-rock 18O values that wer caused by meteoric-water cementation of marine ooids. Sponge-phylloid algal mounds of the protected Llano Platform are also depleted in 18O, which suggests they also underwent meteoric-water diagenesis when the interior platform was exposed to rainfall during sea-level lowstands. Deductions about original mineralogy are based on several indirect lines of evidence. For example, high Sr:Mg ratios in several of the interior platform facies may reflect local concentrations of precursor aragonite. Celestite inclusions in microspar and chert, fluorite inclusions in microspar, and needle-mud pseudomorphs (calcite after aragonite) in matrix are additional by-products of the aragonite-to-calcite transformation in the sediment in the interior-platform environment. Microspar is common in sponge-algal biomicrites of the interior platform where an aragonitic sediment precursor underwent varying amounts of meteoric-water diagenesis. The microspar grades texturally into isotopically depleted spar which is interpreted to have had a meteoric origin. The microspar formed without a micrite precursor by aragonite solution and calcite precipitation in aragonite muds. This occurred in relatively fresh, but highly exchanged and reducing fluids. The replacement of aragonite by microspar was really a dissolution-reprecipitation process in which strontium was retained in the bulk sediment. Micrite, lacking strontium, may have formed more slowly, even after stabilization was complete. The Marble Falls escaped burial and later diagenesis.

Depositional Systems and Stratigraphic Relationships of Strawn Group (Pennsylvanian), Colorado River Valley, Central Texas: ABSTRACT

The Strawn Group (late Atokan-early Missourian) comprises a thick sequence of terrigenous clastic and carbonate facies that crop out in central Texas in two roughly triangular areas separated by Cretaceous overlap. The exposures in the Brazos River valley have been studied in detail and interpreted as fluvial, deltaic, and shallow marine deposits. The Strawn Group of the Colorado River valley has been cited in the literature, but little detailed work regarding subdivision or interpretation has been done. The Strawn Group exposed in the Colorado River valley can be informally subdivided into two general stratigraphic units. The lower Strawn represents submarine fan and basin depositional systems that prograded southwestward. At least two but possibly several cycles of fan progradation are recognizable based on vertical succession of distal to more proximal middle-fan facies associations. In marked contrast, the upper Strawn represents three regressive and transgressive cycles of typical Pennsylvanian deltaic, perideltaic, and shallow submarine depositional systems. Stratigraphic relationships of the Strawn Group are complex and reflect changes in the evolving Ouachita foldbelt and its associated tectonic features. The lower Strawn occurs only in the Fort Worth basin and apparently was deposited with a clinoformal relationship to the underlying basinal Smithwick Shale. It records foreland basin subsidence and regional block faulting synchronous with orogenesis. As active tectonic subsidence shifted westward, upper Strawn deltaic systems prograded to the west, onlapped the Concho platform, and truncated older units particularly on horst blocks. In the southeastern portion of the Concho platform, the overlying Canyon Group completely onlaps the Strawn Group and rests unconformably on the Marble Falls Limestone. End_of_Article - Last_Page 147------------

Depositional Environments of Pennsylvanian Upper Strawn Group in McCulloch and San Saba Counties, Texas: ABSTRACT

Upper Strawn Group (Desmoinesean) represents a transition to fluvial facies from progradational deltaic facies. The lower part of the upper Strawn is composed mostly of horizontally bedded, fine-grained sandstones and shales of a distal delta-front origin. These sandstones and shales exhibit foreset bed dips of up to 15°. In addition to the dipping foreset beds, the delta-front facies on occasion contain small listric normal faults, resulting from periodic higher rates of sedimentation. The middle parts of the upper Strawn consist predominantly of massive, fine to medium-grained, mature sandstones which represent distributary-mouth-bar deposits, as well as other proximal delta-front deposits such as distributary channels. The upper part of the upper Strawn consists o fluvial trough cross-bedded sandstones and chert-pebble conglomerates. These overlie the deltaic facies and indicate the final stages of upper Strawn deposition. The upper Strawn is overlain by the Adams Branch limestone and shales which represent marine transgression and subsequent shallow-marine deposition. The upper Strawn Group in McCulloch and San Saba Counties, Texas, represents continued filling of the Fort Worth basin during Desmoinesean time. The upper Strawn overlies the lower Strawn, an older, deeper water facies, in most parts of the study area. The upper Strawn overlies the Atokan age Marble Falls Limestone in an isolated section of the study area due to its position there on the Concho arch. End_of_Article - Last_Page 489------------

Oxygen Isotopes as Index for Paleosalinity During Deposition of Pennsylvanian Marble Falls Limestone of Central Texas: ABSTRACT

One major goal of a depositional model is to portray variations in salinity across an ancient facies tract. Qualitative determination of paleosalinity gradients with whole-rock, oxygen isotope analyses of limestones is usually hampered by the effects of cementation and reequilibration with formation water at depth. Numerous characteristics of the Marble Falls Limestone make it ideal for facies analysis using isotopes. Burial did not exceed 1 km and the low vitrinite reflectance, averaging 0.30%, indicates cool temperatures. A closed system during stabilization is manifested in high strontium levels in calcite (2,000 to 5,000 ppm). The Marble Falls was deposited on the Llano platform which was bordered on the east by the Fort Worth basin. An east-west sediment profile consisted of black spiculite, algal boundstone, oolitic grainstone, spiculitic wackestone, and calcareous shale. Carbonate mud within spiculite and boundstone of the foreslope environment is isotopically (^dgr18O = -2.52 + 0.55 per mil PDB, n = 12). The whole-rock composition of grainstone (^dgr18O = -4.00 ± 0.56, n = 10) is in accord with a two-component mixture consisting of marine ooids and meteoric cement, supporting the contention that the platform rim was exposed. The high standard deviation of the del values for spiculitic wackestone from the platform interior (^dgr18O = -3.76 ± 1.24, n = 2 ) is due to changing amounts of runoff from the Concho arch. Calcite within the shale, a marsh deposit, is depleted (^dgr18O = -5.10 ± 2.26, n = 2). It is rare to find limestone sequences with facies-specific oxygen isotopes because most have undergone complex cementation histories or burial-related recrystallization. In limestones known to have escaped burial, oxygen isotopes should be of great utility for paleosalinity determinations, when combined with other types of data. End_of_Article - Last_Page 642------------

Stratigraphy and Exploration Geology of Marble Falls Group, Llano Uplift to Southern Part of Fort Worth Basin: ABSTRACT

The Marble Falls Group is a Lower Pennsylvanian carbonate complex that crops out in a discontinuous arcuate belt rimming the east, north, and west sides of the Llano uplift. The carbonate beds accumulated on the eastern part of the Texas craton, which sloped toward the adjacent Fort Worth basin. Three units within the Marble Falls Group have been mapped in the outcrop area. They are informally referred to as the lower middle shale, and The same tripartite subdivision is evident in the subsurface north of the Llano uplift in Lampasas, Mills, Hamilton, Comanche, and Brown Counties. Marble Falls gas production in the southern part of the Fort Worth basin is almost exclusively from the upper limestone. The upper Marble Falls forms several northeast to southwest-trending carbonate-bank complexes. The bank complexes terminate abruptly and pass laterally into shale and dark spiculitic limestone. Both structural and stratigraphic traps are evident within the bank complexes. Pottsville field in Hamilton County is a steep-sided structural trap from which 10 wells have produced approximately 33 Bcf of gas at depths of 2,600 to 2,900 ft (792 to 884 m). Santa Anna field in Coleman and Brown Counties is a large stratigraphic trap discovered in 1928. More than 100 gas wells have been completed in Santa Anna field. Early production records are not available. However, the inco plete data that are published substantiate the fact that Santa Anna is a large, economically attractive field, in which many wells have produced at least 1 to 2 Bcf of gas at depths of 2,300 to 2,400 ft (700 to 732 m). Porosity tends to develop within three facies of the upper Marble Falls limestone: phylloid algal limestone, stromatoporoid limestone, and oolitic carcarenite. At the outcrop, the algal, stromatoporoid, and oolitic facies are most prevalent near the edge of bank complexes. However, they are by no means restricted to the outer part of the banks. Fractures related to a system of mostly down-to-basin normal faults enhance permeability in many places. Structure is complicated and difficult to perceive without seismic data. Even where seismic data are available, standard isopach mapping techniques cannot be employed because there are no continuous shallow seismic reflectors. Moreover, topographic highs capped by Edwards Limestone outcrops tend to yield poor-quality records. In spite of the problems inherent in exploring a structurally and stratigraphically complex area, there are undoubtedly undiscovered, commercially attractive gas fields in the southern part of the Fort Worth basin. The high exploration risk is to some extent offset by shallow depth, low acreage costs, and the attractive nature of potential targets. End_of_Article - Last_Page 1427------------

Subsurface Study of Atoka (Lower Pennsylvanian) Clastic Rocks in Parts of Jack, Palo Pinto, Parker, and Wise Counties, North-Central Texas

A lithofacies analysis of the subsurface Atoka clastic rocks in parts of Jack, Palo Pinto, Parker, and Wise Counties, north-central Texas, was designed to provide a meaningful interpretation of depositional environments that existed during Atokan time, as well as the most favorable areas for production of hydrocarbons. The Atoka clastic rocks are the product of the Ouachita orogene interacting with an epeiric sea on the west during Early Pennsylvanian time. They are a series of sediments deposited between the Caddo Limestone of Strawn age and the top of Marble Falls Limestone of Morrowan age. The study area is in the west-central part of the Fort Worth basin which is a pericratonic or retro-arc basin in plate tectonic terminology. The deepest part of the basin is adjacent to the folded and thrust-faulted Ouachita belt on the southeast. The subsidence of the Fort Worth basin during Atokan time accommodated the eroded detritus from the rising Ouachita system on the southeast. Limestones were accumulating in the western area where more stable conditions existed on the Concho platform. The investigation provided valuable information that may augment hydrocarbon exploration programs throughout the basin; the procedure of investigation can be applied to similar basins beyond the confines of the immediate area.

Early Diagenetic Chert in the Marble Falls Group (Pennsylvanian) of Central Texas

ABSTRACT Chemically-precipitated quartz, confined to a thin interval at an unconformity within the Marble Falls Limestone, originated during an episode of Early Pennsylvanian subaerial exposure. The chert is principally early diagenetic chalcedony and microcrystalline quartz that formed as pore-filling cement and as replacement products for carbonate allochems. Preservation of incipient carbonate cement, stabilized at the margins of pore spaces when microquartz was precipitated in pore centers, and reworked siliceous allochems provide the principal evidence for early diagenetic silica emplacement. Corrosion surfaces in carbonates at the base of siliceous intervals indicate silica was precipitated from pore fluids undersaturated with respect to CaCO3. Organic matter preserved in microquartz probably played an important role in precipitation of silica by locally reducing silica solubility and promoting precipitation.

Biostratigraphy of Carboniferous Rocks, San Saba County, Central Texas: ABSTRACT

Increased understanding of complex carbonate facies patterns in Carboniferous rocks of central Texas permits systematic investigations of macropaleontology and biostratigraphy. Fossils studied were collected from exposures of Chappel, Barnett, Marble Falls, and Smithwick formations in the north-central part of the Llano uplift. The first purpose of this investigation was to identify and describe the macrofauna as completely as possible. Brachiopods, particularly productids and chonetids, dominate the fossil assemblages. However, colonial and solitary corals, bryozoans, gastropods, pelecypods, goniatites, nautiloids, crinoids, and a few trilobites are also present. A second purpose of the investigation was to assess relations among faunas and the several different carbonate and shale facies. For example, the fauna preserved in Chappel beach sediment is completely different from the fauna preserved in conformably overlying Barnett Shale. Most of the taxa from the Barnett range upward into the Marble Falls Limestone, where initiation of carbonate sedimentation resulted in greatly increased fossil abundance and species diversity. This population explosion was halted by the rapid influx of prodeltaic Smithwick mud. An examination of the faunas across the Mississippian-Pennsylvanian and Morrowan-Atokan boundaries determined that changes within specific groups were adaptations to different ecologic conditions. End_of_Article - Last_Page 849------------

Pottsville Gas Area of Hamilton and Comanche Counties, Texas: ABSTRACT

The Pottsville Gas Area, as it is defined in this paper, is geographically located 90 miles southwest of Fort Worth in Hamilton County and southern Comanche County, Texas. Geologically, it is in the west part of the Fort Worth basin approximately 20 miles east of the Bend flexure. The Marble Falls limestone in the Atoka series of the Middle Pennsylvanian is the gas-producing formation of the area. This limestone occurs in an area 25-35 miles wide trending northeast from its outcrop on the Llano uplift in San Saba County. Under the Pottsville gas area the Marble Falls limestone is deposited on top of the Comyn limestone and is encountered at depths ranging from 2,369 feet to 3,850 feet. The Energy field of Comanche County and the Pottsville and South Pottsville fields of Hamilton County are Marble Falls gas fields located in the west part of the Pottsville gas area. These fields overlie adjacent anticlinal structures developed on a horst which was formed by post-Marble Falls forces associated with the Llano uplift. Structural relief on the Marble Falls in these fields ranges from 300 feet in the Energy and South Pottsville fields to 500 feet in the Pottsville field with synclinal troughs separating the individual features. Exploration in the Pottsville gas area for undiscovered fields of the Energy, Pottsville, and South Pottsville type should be confined to the depositional limits of the Marble Falls limestone. Seismograph work guided by subsurface control appears to be the most efficient method to explore for these traps. End_of_Article - Last_Page 124------------

Lower Pennsylvanian Terminology in Central Texas

Presence of fusiform fusulinids in Marble Falls limestone indicates this formation is post-Morrow in age. Lampasas is not appropriate as a series name, because in its type area it is only partly exposed and because as defined it includes beds of Des Moines age and excludes part of the Marble Falls, which is post-Morrow in age. Elevation of the Atoka formation of Oklahoma to Atoka series and use of this term in Central Texas is proposed.

Laboratory Investigations on Acid Treatment of Oil Sands

The practice of introducing acid into oil wells to increase production ofoil and gas has been in use since 1894, when it was first used in thePennsylvania oil fields. It is only since 1928 that it has been used to anyextent in the Mid-Continent. The process has proved successful in increasingproduction, at least temporarily, in depleted oil fields where the producingformation is highly calcareous and where water drive is not too pronounced. Inthe Zwolle field in Louisiana, where the producing horizon is a chalk, increases of 100 per cent have been recorded In north-central Texas, where theoil is produced from the Marble Falls limestone, equally pronounced increaseshave resulted. The production also in some fields yielding oil from quartz sandcemented with calcite has been benefited. The evidence of actual increase intotal recovery of oil, however, has been questioned by some engineers. It hasbeen pointed out that the results of the treatment of a well in a new district, where the process has never been tried out, can never be predicted withcertainty, since other factors besides percentage of calcareous content of theoil sand playa large part in determining the success of the method. Numerous articles have been published on acid treatment and on the effects ofacid on limestone, yet very little has been written concerning experimentalwork on the actual effect of acid on capillary pores and larger oil-coatedvoids of oil sands. This is due partly to difficulties in duplicating fieldconditions in a laboratory and partly to the fact that the process has beencarried out largely by contracting companies that have been unwilling topublish details of their results for fear of loss of business to competitors.The authors, using equipment developed to measure the radial permeability ofoil-sand cores, have developed a method of investigating the results of acidtreatment which has enabled them to test numerous cores and to study in somedetail the action of acid on the different types of rocks, particularlyfine-grained rocks.