Sand-shale Ratio Research Articles

Calculation of Thermal Conductivity from Lithology and Laboratory Measurements on Oil Well Cores: ABSTRACT

The harmonic mean thermal conductivity of the whole rock column traversed by a well is calculated from the thicknesses of the different kinds of rock in the section and their respective thermal conductivities. Lithologic logs provide the thicknesses; laboratory measurements on cores, the conductivities. Heat flow can be then calculated by multiplying the mean conductivity by the temperature gradient obtained from corrected bottom-hole temperatures. In the sedimentary basins of Sumatra and Brazil the conductivities range from 4.1 to 6.5 mCal/cm°C sec but the average for all the basins is close to 5. The mean conductivity of a well depends on the sand-shale ratio and can probably be calculated without loss of precision without recourse to more measurements on samples. onductivity determinations and heat flow in three Tertiary basins in Sumatra and in two Cretaceous basins in Brazil involving over 2,000 wells are reviewed. It is suggested that calculations of mean conductivity and heat flow be added to the existing AAPG compilation of temperature gradients in North America. End_of_Article - Last_Page 1364------------

Stratigraphy of the Jurassic – Lower Cretaceous sediments of Jameson Land and Scoresby Land, East Greenland

The Jurassic-Lower Cretaceous sediments of Jameson Land and Scoresby Land are divided into seven formations. They comprise from bottom to top: Kap Stewart Formation of Rhaetian-Liassic age; Neill Klinter Formation of PliensbachianToarcian age divided into three members; Vardekløft Formation of ?BajocianMiddle Callovian age redefined and divided into three members; Olympen Formation of Upper Callovian-Middle Oxfordian age designated as a new formation; Hareelv Formation of Upper Oxfordian-Middle Kimmeridgian age designated as a new formation; Raukelv Formation of Upper Kimmeridgian-Upper Volgian (Lower Ryazanian?) age designated as a new formation and divided into three members; Hesteelv Formation of Ryazanian age designated as a new formation and divided into two members. The six lower formations together comprise the Jameson Land Group. The main facies types are described. They comprise black shales, shaly siltstones, horizontally bedded sandstones, ripple-laminated sandstones, massive-bedded sandstones, medium- and large-scale cross-bedded sandstones, black shales with lenticular sand-bodies and coals. A general interpretation of the changing depositional regimes throughout the Jurassic and Lower Cretaceous of Jameson Land and Scoresby Land is given. The interpretation is based on the spatial distribution of the different facies types, on longitudinal profiles, and on investigations and measurements of cross-bedding, parting-lineation, grain sizes and sand-shale ratios. The exact geographical position of the borderlands and thus of the source areas of the sediments are rarely known but the predominance of very coarse sediments and the large-scale sedimentary structures indicate a position very near the coast.

A METHOD OF CONTINUOUS VELOCITY ANALYSIS

This continuous velocity analysis technique is a development of previous automated computer methods for extracting velocity information from seismic reflection data. Every trace of the multi-fold coverage data is used to measure the time, amplitude, moveout and dip of each coherent reflection event on the seismic records, by means of moveout scan and dip scan techniques. Using these four parameters, reflection events are tracked along the line, finally producing a computer file on each reflection event, listing its time, amplitude and moveout at each depth point along its extent. The signal to noise ratio increase given by the moveout and dip scan processes is greater than that achieved on the final section, allowing the computer to pick events close to the limit of visibility on the final section. The velocity information obtained can be displayed in various ways, for use in estimating velocity functions for data processes such as moveout correction, time migration, and depth conversion, and for use by the interpreter in such things as the measurement of interval velocities and the estimation of sand-shale ratios.

Paleoecology, Stratigraphy, Production--Getting It All Together in Offshore Louisiana: ABSTRACT

ABSTRACT Statistical studies based on the mapping of paleoecologic zones, sand conditions and production in offshore Louisiana indicate a positive correlation among these parameters. Oil and gas production for any given depth is associated with a narrow range of preferred environments of deposition and sand development. For example, in the depth range 10,000-12,000 feet in the eastern offshore area (Ship Shoal through Main Pass), about 70 percent of all oil completions occur where the section contains 10 to 30 percent sand and is in the inner neritic environment. An IBM 1130 computer with attached Calcomp plotter was used to prepare: 1) structure and environment maps of various biostratigraphic zones, 2) environment maps of the interval between biostratigraphic tops, and 3) environment maps at constant depth slices. To aid in statistical comparisons, the following maps were constructed in 2,000 foot-thick depth intervals: 1) average weighted environment, 2) sand percentage, net sand, sand-shale ratio, and number of discrete sand bodies, and 3) oil and gas completions.

Stratigraphy and Sedimentology of Jackfork Group, Arkansas

The Jackfork Group is a flysch deposit which crops out along the frontal Ouachita Mountains and the Athens Plateau of Arkansas. Along the frontal Ouachitas, the 5,400-ft section is 70 percent shale, most of which is contorted by soft-sediment deformation. The lack of siliceous shale zones within the Jackfork does not permit Oklahoma stratigraphic nomenclature to be used; therefore, two new formations of approximately equal thickness are proposed. On the Athens Plateau, the group is 6,000 ft thick and consists of 70 percent wackes and arenites. Pitkin Limestone exotics in the underlying Chickasaw Creek Formation suggest that the Jackfork was deposited during the formation of the Pitkin-Hale unconformity. Differences in thickness, mineral composition, sedimentary structures, and sand-shale ratios allow the construction of a depositional model. Poorly sorted, indistinctly bedded rocks, considered to have been deposited as delta-front and prodeltaic sediment, are present along the southeastern Ouachitas. Sedimentation appears to have resulted chiefly from suspension and redistribution by weak tractional currents and slumping. Proximal turbidites, which developed very near the inception of turbidity flows, consist of thick-bedded sandstones generally with scoured bedding contacts, and are present throughout the eastern parts of the belts. Distal turbidites become more common westward, and are thinner bedded, poorly graded, laminated sandstones with abundant hieroglyphs. Beds disturbed and disrupted by soft-sediment deformation developed at the foot of a northern unstable slope. This allochthonous material entered at right angles to the basinal turbidites which flowed westward down the trough axis. Highest sand-shale ratios parallel the axis of the trough, whereas slope deposits contain higher pelitic contents that are commonly disturbed.

Classification and Interpretation of Disturbed Bedding Types in Jackfork Flysch Rocks (Upper Mississippian), Ouachita Mountains, Arkansas

ABSTRACT The Jackfork Group in the Ouachita Mountains of Arkansas is an Upper Mississippian flysch succession that displays readily distinguishable lithofacies. The frontal Ouachitas contain a preponderance of disturbed beds which are interbedded with turbidite sandstones. These disturbed and disrupted beds developed at the foot of a northern unstable slope, the resultant bedding dependent upon the type of material involved, its state of consolidation, and the distance moved. Information gathered concerning the progressive changes from one type of sedimentary structure to another gives a qualitative measure of downslope distance. Muds which undergo submarine deformation upon a sloping sea floor first become flowage shales which change to blocky mudstones with continued failure. Failure of plas ic muds and cohesive sandstone progressively form contorted, chaotic, and rubble bedding. Deformation of muds and sands form slurried bedding and breccias. Movement of flowing, creeping, or sliding clean sand may have occurred but is not considered to be a type of disruptive bedding. The deformation of mobilized, liquefied sand under some sediment cover produced load structures, sandstone dikes, and intrastratal flow structure. The Ouachita trough possibly consisted of an offsetting series of unstable shale slopes deepening southward to a gentle, west-dipping abyssal plain. The muds and channel sands upon the slope frequently slumped southwards, sometimes flowing out over the abyssal floor at right angles to basinal turbidites flowing westward down the trough axis. Belts where disturbed bedding comprises the greatest percentage of the entire section define the unstable margins of the flysch trough while the highest sand-shale ratios parallel the axis of the trough.

Regional Distribution Patterns of Pennsylvanian Sandstones in Illinois Basin

Patterns of Pennsylvanian sandstone deposition in the Illinois Basin are delineated by a series of local and regional maps. Pennsylvanian sand bodies in the basin are classified into sheet and several elongate types. These sand bodies occur throughout the basin. Regional patterns of sandstone deposition are displayed by basin-wide maps of two intervals: from the base of the Pennsylvanian system to the Illinois No. 2 coal and its equivalents and from the No. 2 coal to the Shoal Creek Limestone and its equivalents in the basin. Number of sand bodies, sand thickness, and sand-shale ratios were mapped. All three maps have broadly similar basin-wide patterns. They differentiate the more rapidly subsiding basin from more stable western and northern shelf areas and, in conjunction with two isopach maps and a basin-wide compilation of 1887 cross-bedding directions, reveal the dispersal patterns of clastics into and across the basin. Clastics came from the east, north, and northwest. The salient features of Pennsylvanian sedimentation in the basin are summarized in terms of basin geometry, lithic fill, arrangement or distribution of lithic fill within the basin, and tectonic setting.

Neogene Paleogeography of the Surrounding Dewa Hilly Lands

Oil occurence in basins may generally be related to the relief of basins bottom and to the sedimentary environments. To clarify the process of the oil accumulation, the study of paleography is a important method and a basic information for the oil exploration. The writer studied on the stratigraphy of the Neogene strata developed in the surrounding area of the Dewa hilly lands, which include the southern part of the Akita and Yamagata, the eastern part of Miyagi prefectures, and introduced several time-stratigraphic units and its formations. The strgtigraphical relationships of each province are discussed, and inter-provincial correlation are shown in Table 1. On the study of the isopach map, down ward sinking and upward movement area of the sedimentary basin are clarified. But isopach map alone will not give a trusty aspect of a depositional basin. The lithofacies and the biofacies are equally important method for the restoration of basin depth and sediment-tary environments. Combinations of the lithofacies, biofacies and isopach, make the aspect of a depositional basin clear. Accordingly, on the basis of the ecological study of the fossil foraminifera, the six biofacies are recognized, namely (1) littoral zone biofacies, (2) inner neritic zone biofacies, (3) outer neritic zone biofacies, (4) bathyal zone biofacies, (5) deep stagnant zone biofacies, (5) deep stagnant zone biofacies, (6) shallow stagnant zone biofacies. The writer prepared the paleogeographical maps of the Nishikurosawa stage, Onnagawa stage, Funakawa stage, Kitaura stage and Wakimoto stage, making account of the lithofacies, biofacies and isopach, those are shown in Fig. 18-22. As the result of the study, the Dewa Hilly lands was situated on the subsiding zone during the Onnagawa stage, but in the late Funakawa stage, was slowly uplifted from south to north. Oil occurence is closely related to the paleogeographical aspects. Submarine ridges in the deeper stagnant sea are one of the most favorable places for the oil accumuration. The Amarume and Fukura oil fields may be an example of this relationship. The Innai-Katsurazaka and Horiuchi oil fields are situated in the hinge belt, where the sand-shale ratio is bitween 1/8 to 1.

Problems and Principles of Sandstone Body Classification: ABSTRACT

The geometry of sandstone bodies involves their shape, size, and orientation. The original geometry is subject to later modification by erosion, faulting, folding, tilting, compaction of underlying sediment, and internal compaction. Although orientation or trend has been, and will continue to be used successfully in some cases without knowledge of the origin of the sandstone bodies, greater predictability should be possible if the origin can be determined--provided the distributional patterns of sediments of various origins are known. Insofar as geometry is concerned, three major problems are (1) to reconstruct the geometry correctly, (2) to know what it implies regarding origin, and (3) to know the distributional pattern of sediment of that origin in an analogous depositional situation. For reconstruction of sandstone body geometry, total sand thicknesses or sand-shale ratios for thick sedimentary sequences are of limited value. Isopach maps of individual sand bodies define their size and orientation, but only partially define their shape; cross-sections hung on a closely related underlying or overlying bed, whose original attitude relative to the sandstone body is known or can be reasonably assumed, are also required to define shape. Possible modification of original shape by compaction or other processes must be considered. The plan dimensions of present-day deltas, barrier bars, and other sedimentary types are rather well-known, but three-dimensional data are scarce. Too often, third-dimensional data from ancient sediments are misleading because the origin has been incorrectly determined. Such internal features as cross bedding, flow markings, grain orientation, bed or grain size sequences, and the relationship of a sandstone body to beds above, below, and laterally are important for interpreting origin particularly where control is too sparse to define the geometry. End_of_Article - Last_Page 1256------------

Stratigraphic Analysis of Silurian Rocks in Michigan Basin

Samples from 35 exploratory wells penetrating a complete section of Silurian rocks in the Michigan basin are analyzed by conventional quantitative chemical methods. Chemical analysis yields numerical lithologic data not available from study of published subsurface information. The numerical lithologic data are used to construct clastic ratio, sand-shale ratio, evaporite ratio, and isopach maps which depict gross aspects of Silurian sedimentation, history, and environmental tectonics of the basin. Analysis of the lithofacies maps reveals eight areal subdivisions of the Michigan basin that are interpreted in terms of sedimentation and regional tectonics. Most significant of the eight subdivisions are the Central Michigan basin, East Michigan basin, and Battle Creek trough. Positive elements influencing history of the basin are the Findlay and Kankakee arches. Battle Creek trough was a major link between the Illinois and Michigan basins, but was severed at time of isolation of the Salina sea. Silurian rocks in the Michigan basin have produced only insignificant amounts of oil and relatively minor amounts of gas, but more exploration is indicated. A tectonic and lithofacies hinge across south-central Michigan, separating the Central Michigan basin from a southern shelf area, is suggested as a possible locale for development of important biohermal structures in the late Middle Silurian with resultant petroleum entrapment.

Differential Compaction of Pennsylvanian Sediments in Relation to Sand-Shale Ratios, Jefferson County, Illinois

ABSTRACT In Jefferson County, in the Illinois coal basin, southern Illinois, the Pennsylvanian column between the Shoal Creek and Stonefort Limestones, an interval of about 900 feet, was divided into seven intervals separated by widespread coals or limestones readily recognized on electric logs, namely the Shoal Creek Limestone, coal No. 8, West Franklin Limestone, coal No. 7, coal No. 5, coal No. 4, coal No. 2, and the Stonefort Limestone listed in descending order. These intervals were determined from 578 electric logs in the county, an average of about one per square mile. Sand-shale ratios were determined for each interval and it was found than a channel sandstone of one age occurs in each interval except between the Shoal Creek Limestone and coal No. 8. Two ages of channels occur in one interval. Sandstone percentage maps were made of each interval. Thicknesses of each interval were compared in the channel areas and nearby, outside the channel, to avoid regional variations in thickness. Assuming that the coals and limestones were laid down nearly parallel to each other, variations in interval result from greater compaction of the shales than of the sandstones. Results show the following percentages of differential compaction for the six intervals with channels listed in descending order: 21, 34, 29, 35, 42 and 54 percent.

Differential compaction of Pennsylvanian sediments in relation to sand-shale ratios, Jefferson County, Illinois

Differential compaction of Pennsylvanian sediments in relation to sand-shale ratios, Jefferson County, Illinois

Thickness of Strata

ABSTRACT Thick and applied to bedding are terms that are relative to one's stratigraphic training and experience and relative to the associated strata. The terms, thick bedded, medium bedded, and thin bedded should not be used without definition, although undefined usage is common in the current literature. Recent efforts to standardize the limits of thick, medium, and for bedding are in considerable disagreement. Standardization or pigeon-holing of thicknesses may be necessary only for statistical treatment of stratification. However, in such studies mean bed thickness, thickness frequencies, and other statistical indices may serve to better describe and understand the origin of the assemblage. Many formations possess stratification facies (phyllofacies) as well as biofacies, and lithofacies. It may be just as productive to determine regionally the stratification indices or ratios as it is to study sand-shale ratios. It may be possible and useful to construct regional iso-stratification (isobed) maps just as it is to construct isolith maps. It is proposed that the beddedness of sedimentary units be expressed by the following index. Stratification index =[no. of beds100]/[thickness] The task of determining, measuring, and counting individual beds is large and subjective. However, it may consist of a partial sampling just as sampling for lithology or chemical composition commonly is. I am convinced from my consideration of the quantitative and statistical approach to bed thickness that the additional effort involved yields a much better awareness and appreciation of the nature and significance of bedding than without such an approach.

Statistical Analysis of Facies Maps

Regional facies maps based on one or more sets of contour lines (as clastic ratio and sand-shale ratio) may display well-defined trends, but some show irregular, spotty patterns, difficult to interpret. The latter may arise when local geologic variations are great enough to mask regional trends. The problem of applying statistical analysis to facies maps thus requires use of models suitable to the sorting-out of variations produced by geologic factors. Analysis of variance provides basic techniques for this purpose. Single-factor and two-factor analysis of variance designs are useful in testing maps for "facies homogeneity" by comparing variation within areas with variation between areas. In such applications, the statistical designs are used to detect relationships between fixed means. The methods are illustrated on a regional scale by Pennsylvanian data in Kansas, which show a clastic ratio gradient but no sand-shale ratio gradient. On a local scale the method is used to test for significant differences in stratigraphic aspect within a single pool.

General Lithofacies Relationship of St. Peter Sandstone and Simpson Group

Stratigraphic relationships and area of distribution of the St. Peter sandstone and the Simpson group indicate that the two constitute an extensive and continuous sheet of clastic rocks interlayered with equally extensive bodies of carbonate strata. Certain bulk physical properties of the clastic sheet, such as sand-shale ratio, number of sands, and thickness, increase systematically into southern Oklahoma, the thickest part of the Simpson basin. Except for local thickening in Illinois, the St. Peter shows no regular changes in percentage of shale, number of sandstone beds, or grain size, but it is strikingly uniform. In southern Indiana and western Kentucky, the sandstone sheet grades abruptly into carbonates without intervening shales; whereas, in the Simpson basin abun ant shales characterize the section. Here intertonguing of shale and sandstone is noteworthy. Particle-size distribution of sandstone of the Simpson parallels that of the St. Peter and suggests genetic relationship of the source material. Distribution and position of the shale and carbonates indicate a northern source of clay and sand debris. Carbonates of the Dutchtown and Joachim which overlie the St. Peter in an area of southern Illinois, Missouri, and Indiana indicate that sand deposition migrated northward as these carbonates accumulated. A generally northward transgression of shorelines is interpreted. The St. Peter is theorized to be a continuous series of coalescent shoreline sands migrating over a stable shelf. Simpson deposition represents transgression and local regression of shorelines spread outward from the major part of the subsiding basin.

Pennsylvanian Rocks of Eastern Interior Basin

Pennsylvanian rocks of the Eastern Interior basin underlie about 53,000 square miles in Illinois, Indiana, and western Kentucky, with small outliers in Missouri and Iowa, attaining maximum thickness of about 2,500 feet in southeastern Illinois. At the beginning of the Pennsylvanian there was a regional southwest slope furrowed by numerous subparallel valleys as deep as 200 feet. An eastward slope prevailed along the western border of the basin with smaller valleys. Subjacent strata range in age from the Middle Ordovician St. Peter sandstone to the Upper Mississippian Kinkaid limestone. Because of topographic unevenness and early Pennsylvanian tectonism, the early Pennsylvanian Caseyville and Tradewater units vary from zero to 1,200 feet. Movements along the LaSalle anticl ne and Duquoin monocline are reflected in an isopach map of this interval. In later Pennsylvanian Carbondale and McLeansboro time the intervals increase regularly toward the southeast, showing that the present southeast border of the basin is due to post-Pennsylvanian deformation. Structural relief on the No. 2 coal ranges from +700 to -913 feet. Similar systems of nomenclature are used in Illinois and Kentucky but different names are employed in Indiana. The significant features of 46 key beds are described. Clastic ratios average about 8 parts clastic to 1 non-clastic. Sand-shale ratios are extremely variable due to lenticular sandstones at more than 20 positions. Many sandstones occupy channels excavated into underlying strata; others seem to be off-shore barrier beach deposits. There are marine limestones at about 25 different positions, generally thickening toward the western border of the basin, but each has its individual pattern of distribution. Lower Pennsylvanian sandstones are highly quartzose and derived from older sediments; later sandstones are micaceous and feldspathic with much interstitial clay, evidently a first-cycle deposit from a metamorphic terrane. Coals are generally banded, and range in rank from high-volatile to medium-volatile bituminous. Refractory underclays ar found in the Lower Pennsylvanian in northern Illinois and in the area near St. Louis, Missouri. Cyclic sedimentation is very well displayed with at least 25 alternations of coal, marine limestone, shale, sandstone, and underclay. Early Pennsylvanian sediments came from the east and northeast but the source of the later micaceous sandstones is still uncertain. Most marine invasions came from the west, north of the Ozark uplift, but some in the early Pennsylvanian probably entered the basin from the east and south. The basin has produced about 5 billion tons of coal and 325 million barrels of Pennsylvanian oil. Other industries based on Pennsylvanian materials are refractories, common brick and tile, road metal, agricultural limestone, portland cement, rock asphalt, glass sand, foundry sand, and whetstone.

Stratigraphic Analysis of Upper Devonian and Mississippian Rocks in Michigan Basin

Upper Devonian and Mississippian rocks in the Michigan basin are analyzed by subsurface methods. Sections from one hundred sixty-eight oil wells were studied. Isopach and lithofacies maps serve as a basis for interpreting sedimentation processes, tectonics, and environmental conditions. In order to work out a more complete picture of Upper Devonian and Mississippian sedimentation, it is necessary to subdivide the total section into operational rock units A, B, and C, which are recognized on the basis of lithologic criteria. Respective units are then interpreted in terms of tectonic and environmental conditions responsible for patterns exhibited. Integration of three arbitrary units of Upper Devonian and Mississippian sedimentation presents the regional geologic history. Sand-shale ratio and per cent carbonate or evaporite in the total stratigraphic unit are used in the statistical analysis. Interpretation of isopach and lithofacies patterns reveals that tectonics of the depositional area is a very important factor in controlling sedimentary facies. Tectonic conditions from shelf to open basin, to silled basin, are represented.

Composition of Crude Oil and Its Relation to Stratigraphy in Wyoming

Although the crude oils of Wyoming vary widely in their composition, nearly all of them can be classified into two major groups on the basis of the average composition of oils from each formation. These two groups of oils are found in rocks representing different environments of deposition. The high-sulphur, low-gasoline, aromatic-naphthene base crudes are associated with thick limestone, dolomite, and shale sequences which were deposited in broad shallow seas under conditions of prolonged crustal stability. The low-sulphur, high-gasoline, paraffin-naphthene base crudes are associated with sand-shale sequences deposited in areas of moderate tectonic activity with high rates of subsidence and deposition. Differences in oils within a formation are correlative with differenc s in environment of deposition and with depth of burial. In the Pennsylvanian-Permian formations of Wyoming the oils associated with the dolomite-evaporite facies at the east are more aromatic and naphthenic than the oils associated with the dolomite-shale facies at the west. Within the Frontier formation the oils from areas of low sand-shale ratios contain more paraffins and less sulphur than oils from areas of high sand-shale ratios. Oils from depths greater than 8,000 feet in the Tensleep sandstone of Wyoming generally contain more than 50 per cent gasoline, whereas oils from depths less than 3,000 feet in the sand generally contain less than 25 per cent gasoline. However, there is no change in the paraffinicity of the heavier fractions of the Tensleep oils with depth. Also, there is o correlation between oil composition and depth for oils from all the Wyoming formations. No correlation was found between the composition of the Wyoming oils and their age, or the possible catalytic activity of their reservoir rocks.

Regional Stratigraphic Analysis of Middle and Upper Marine Jurassic in Northern Rocky Mountains-Great Plains

Middle and upper marine Jurassic strata are traced throughout Wyoming, Montana, adjoining parts of Idaho, southern Alberta, Saskatchewan, Manitoba, and western North and South Dakota. Lithologic units recognizable in Montana are used as a basis for tracing areally the various stratigraphic units recognized in the area under consideration. The total section is divided into four recognizable operational units which from oldest to youngest are: Piper, Rierdon, Preuss, and Swift. Tectonic and environmental conditions during middle and upper marine Jurassic time are reconstructed from isopach-lithofacies and paleogeologic maps. Clastic ratios and sand-shale ratios are used to define the lithologic variation. The important tectonic elements which are active during middle and upper marine Jurassic sedimentation include: the arcuate Idaho-Utah basin centered in southeastern Idaho; Belt island of central and southwestern Montana; central Montana and Wyoming shelf area; and the basins in southern Saskatchewan and western North Dakota. Normal marine and some evaporite sediments in the lower part are characteristic of the Piper unit. The Rierdon consists predominantly of normal marine clastics and some carbonates. Coarse clastics make a prominent appearance in the Preuss unit associated with a somewhat restricted environment. Normal marine conditions again prevailed with the deposition of coarse and fine clastics in the Swift.

Petroleum Exploration in Eastern Arkansas: ABSTRACT

The area with which this paper is concerned lies in the Gulf Coastal Plain in Arkansas north of the Arkansas River. The following conclusions are based on a study of the available samples and electric logs. Tertiary rocks are for the most part non-marine in origin. Some of the beds in the Jackson and Claiborne groups may be thin tongues of either marine, deltaic, lagoonal, or estuarine deposits. The Wilcox group is predominantly thick, coarse-grained sandstones which are ordinarily water-bearing. The Midway group consists of two formations: the Porters Creek clay and the Clayton. None of the Tertiary rocks is considered promising as a potential oil reservoir. In the Cretaceous two formations offer possibilities for oil and gas: the Nacatoch sandstone and a basal transgressive sandstone, probably Ozan in age overlying the Paleozoic floor. The Nacatoch is of sufficient thickness and porosity to serve as a reservoir bed. However, local variations in porosity or in the sand-shale ratio should be expected. The basal sand is coarse- to medium-grained, commonly pyritic and glauconitic. Electric logs show a well developed self-potential curve in this unit. If found on structure or as a wedge-edge pinch-out, this basal sand has good possibilities as a future oil source. Paleozoic rocks, ranging in age from Pennsylvanian to Cambro-Ordovician, are present below the Cretaceous. A paleogeographic map of the pre-Cretaceous surface shows that older Paleozoic End_Page 2068------------------------------ formations (Plattin, et cetera) are found as far south as Cross and Crittenden counties. This indicates a marked change in strike of the truncated older rocks. With sufficient cover these older beds may be excellent oil traps. The St. Peter sandstone is considered a particularly good possibility. There is also a possibility that oil traps may be associated with intrusive igneous bodies similar to the nepheline syenite plugs near Little Rock. End_of_Article - Last_Page 2069------------