Sand Lenses Research Articles

Enhanced Gas Recovery From Watered-Out Reservoirs: ABSTRACT

Major additions to future gas reserves will depend partly on development of resources that are presently unconventional. Recently, coproduction of gas and water has been proposed as a method to increase ultimate gas recovery from reservoirs that have begun to water out or were abandoned because of high water production. The enhanced recovery method involves producing large volumes of water and reducing reservoir pressure to the point where dispersed gas, trapped in the water-invaded zone, is mobilized and flows to the well bore. Research involving the coproduction method was funded by the Gas Research Institute. Integration of geological and engineering studies indicates that the Port Arthur field in Jefferson County, Texas, is a prime area for research to test the coproduction techniques. Prior to abandonment, the field produced from lower Hackberry (Oligocene) sandstones at depths less than 12,000 ft (3,600 m). The field contains 11 reservoirs with a maximum net sandstone thickness of over 450 ft (135 m). Individual sandstones are 10 to 120 ft (3 to 36 m) thick, but gas columns are usually less than 20 ft (6 m) thick. The reservoirs are relatively narrow and elongate and are oriented nearly perpendicular to the local structure, a simple roll-over anticline. These reservoirs, interpreted as submarine channel and fan deposits, are internally heterogeneous owing to the vertical imbrication and dip-alignment of the sand lenses. Optimum reservoir facies occur as thick, massive sandstones with uniformly high porosities and permeabilities that average 28% and 60 md, respectively. Reservoir quality is more variable and diminishes somewhat away from the channel axes in the more thinly bedded sandstones with intercalated shales. Upward decreases in porosity and permeability characterize the zones of gas accumulation. The C sandstone was selected for detailed investigation because of high abandonment pressure, excellent reservoir quality, high productivity, and good lateral continuity. Reservoir simulation studies suggest that 5.5 bcf of gas and some condensate could be recovered from the C sandstone under natural flow conditions by producing slightly more than 9 million bbl of water over a 5-year period. Brine disposal would be accomplished by injection into thick Miocene sandstones that occur at shallow depths. If successful, the recovery efficiency of the C sandstone would be increased from 42% (primary) to 53% of the original gas in place. An economic analysis of the enhanced recovery project indicates a favorable payout under prevailing gas prices. Project economics are even more attractive if gas prices increase or if gas production from other abandoned reservoirs is commingled with production from the C sandstone. End_of_Article - Last_Page 475------------

Sedimentology and palynology of Middle Wisconsinan deposits in the Pecatonica River valley, Wisconsin and Illinois

The bedrock valley of the Pecatonica River north of Freeport, Illinois, contains a thick valley-fill complex of alluvium and drift. Within the valley, loess-capped benches surround hills of silty Illinoian drift. Beneath these benches lie thick deposits of poorly sorted stony silt interbedded with thin lenses of silt, sand, and organic-rich loam. Channel deposits and peat cap the diamicton in places. We interpret the stony silts as solifluction debris shed from silty slopes within the valley-fill during the Early or Middle Wisconsinan (Altonian). Top and bottom radiocarbon dates from a 2.5-m section of peat overlying the diamicton are 26,820 ± 200 and 40,500 ± 1700 yr B.P., respectively. We informally refer to the stony silts, channel sediments, and peat as the “Martintown unit.” Geomorphic position, sediment input, and macrofossils suggest that the dated peat was deposited in a floodplain pond (oxbow?). The pollen record from the peat indicates that a boreal forest dominated this area during the Middle Wisconsinan (late Altonian and Farmdalian). Two pollen zones are recognized: a basal Zone I with Pinus slightly more abundant than Picea and with few herbs and shrubs, and an upper Zone II dominated by Picea and with a larger representation of herbaceous and shrub taxa. Little displacement of vegetation zones is indicated, even though ice advanced to within 100 km of the site during the time of peat accumulation. Because of the problems involved in clearly defining Middle Wisconsinan forest-tundra in mid-latitudes by using analogs of Holocene forest-tundra in high latitudes, caution is required in making geomorphic inferences solely from vegetation data. Together, though, pollen and sediment data indicate that during the Middle Wisconsinan, Pecatonica hillslopes progressed through a sequence of instability-stability-instability related to climatic fluctuations.

Influence of Common Sedimentary Structures on Fluid Flow in Reservoir Models

Summary This paper summarizes the state of the art of deriving detailed permeability-distribution models on the basis of cores, sidewall samples, and logs. Reservoir heterogeneities such as clay drapes and intercalations, crossbedding, sand laminations, slumping and burrowing in various major depositional environments are examined. Introduction The rapid increase in the number of secondary- and tertiary-recovery projects has resulted in a greater need for detailed geological modeling. Especially when expensive chemicals are injected into a reservoir, one wants to know how they will be distributed away from the wellbore. Nearly all reservoirs are heterogeneous to such a degree that realistic reservoir simulation cannot be carried out on the basis of homogeneous prototypes. The overall heterogeneity can be dominated by open or sealing faults, fractures, contrasting lithologies, diagenesis, or sedimentological complexity. The influence of such features is described in a relatively large number of papers, while there are few that have concentrated on the internal heterogeneity within a genetically defined sand body caused by sedimentary structures and intercalations. There is a fair understanding of the overall shape and continuity of some of the more important genetic reservoir sand-body types. At the other end of the scale, use of the scanning electron microscope (SEM) has revealed much information on pore shape and diagenesis. This paper focuses On the sedimentary heterogeneities on a scale between an entire, genetically defined, sand body and a small volume of matrix. Although more realistic than glass packs, very homogeneous sandstone for flow and displacement experiments in laboratories - Berea sandstone in the U.S. and Bentheim sandstone in Europe - has been detrimental to the understanding of the influence of medium-scale heterogeneities. There are few examples in the literature of experiments taking into account sedimentary structures and clay laminations. Pettijohn et al1 give good annotated references of such literature and also discuss the hierarchical sequence of primary controls on permeability; They state that because cross-bedding with preferred orientation is the dominant structure in many, if not most, sand reservoirs, it is the prime suspect for depositional control of anisotropic horizontal permeability in a reservoir. The use of internal directional structures for predicting the trend or orientation of sand bodies is beginning to be well understood. Numerous examples are given by Potter and Pettijohn.2 There is nothing new in the statement that the internal permeability distribution in a sand body Will influence fluid flow and sweep efficiency. A literature survey yielded several early papers describing flow experiments on heterogeneous sand models. Mills carried out such experiments as early as 1920.3 He was particularly interested in the trapping of oil in fine-grained sand lenses embedded in coarser sand during a waterflood. It is stated that there is first a rapid movement of water along the paths of least resistance under the laws of hydraulic flow of liquids, followed by readjustment under the laws of capillarity. Unfortunately, the understanding of the influence of sedimentary structures of the scale used in the experiments has not advanced much beyond this statement of 60 years ago. A second early (1939) paper worth mentioning is by Illing.4 He states that the most difficult zone to flush with water is the coarse/fine interface; thus, where there are numerous textural changes in the reservoir rock, it will be increasingly difficult to obtain a high percentage of oil extraction. All such coarse/fine interfaces tend to lock up oil in the strata.

An Exploration Model for Medina Group Sandstones of Western New York and Northern Pennsylvania: ABSTRACT

Hydrocarbon accumulations in the Medina Group of western New York and northern Pennsylvania are controlled by stratigraphic traps. The discontinuous productive sand lenses were previously considered to be unmappable due to the erratic fluviatile deposition and marine reworking of the deltaic sediments. Wells were subsequently drilled only on the basis of pipeline availability, government spacing regulations, and general geology. Many of the completed wells proved to be marginal or noncommercial. Using core analysis, log data, and well production histories on several thousand wells, an exploration model has been developed to improve Medina well success ratios and performance. Specific sedimentary structures can be identified using characteristic gamma-ray patterns and they can be mapped in the subsurface. Highly productive coarsening-upward channel and bar-sand sequences can be projected into undrilled acreage, thereby reducing the percentage of non-economic wells drilled into the Medina sands. End_of_Article - Last_Page 1165------------

Canyon Creek: A late Pleistocene vertebrate locality in interior Alaska

The Canyon Creek vertebrate-fossil locality is an extensive road cut near Fairbanks that exposes sediments that range in age from early Wisconsin to late Holocene. Tanana River gravel at the base of the section evidently formed during the Delta Glaciation of the north-central Alaska Range. Younger layers and lenses of fluvial sand are interbedded with arkosic gravel from Canyon Creek that contains tephra as well as fossil bones of an interstadial fauna about 40,000 years old. Solifluction deposits containing ventifacts, wedge casts, and rodent burrows formed during a subsequent period of periglacial activity that took place during the maximum phase of Donnelly Glaciation about 25,000–17,000 years ago. Overlying sheets of eolian sand are separated by a 9500-year-old paleosol that may correlate with a phase of early Holocene spruce expansion through central Alaska. The Pleistocene fauna from Canyon Creek consists of rodents (indicated by burrows), Mammuthus primigenius (woolly mammoth), Equus lambei (Yukon wild ass), Camelops hesternus (western camel), Bison sp. cf. B. crassicornis (large-horned bison), Ovis sp. cf. O. dalli (mountain sheep), Canis sp. cf. C. lupus (wolf), Lepus sp. cf. L. othus or L. arcticus (tundra hare), and Rangifer sp. (caribou). This assemblage suggests an open landscape in which trees and tall shrubs were either absent or confined to sheltered and moist sites. Camelops evidently was present in eastern Beringia during the middle Wisconsin interstadial interval but may have disappeared during the following glacial episode. The stratigraphic section at Canyon Creek appears to demonstrate that the Delta Glaciation of the north-central Alaska Range is at least in part of early Wisconsin age and was separated from the succeeding Donnelly Glaciation by an interstadial rather than interglacial episode.

Gas Recovery From Tight FormationsA Function of Technology and Economics

Summary Three major findings emerge from the study of the technology and economics of recovering natural gas from tight gas formations.1. A minimum threshold price, near $3.00/Mcf (in mid-1977 dollars) and equivalent to approximately $4.50/Mcf (in mid-1981 dollars), is required to make the tight gas resource attractive for investment.2. Beyond the threshold price, improvements in technology have a greater impact in terms of gas recovery than further increases in gas prices.3. The combination of increased gas prices and improved recovery technology provides the most effective means for accelerating production from this vast domestic natural gas resource - calculated at more than 400 Tcf in place.The technology improvements that have the largest impact on ultimate recovery are (1) improved ability to identify the net pay and characterize the reservoir, (2) capacity to stimulate multiple reservoirs from a common wellbore, (3) increased effective fracture length, (4) ability for fractures to intersect sand lens beyond the wellbore, and (5) optimization of field development. Background Large quantities of natural gas exist in tight (low-permeability) formations where the gas flow is too low for economical recovery by means of conventional technology. The basins containing these formations stretch westward from the Cotton Valley trend in Louisiana, through Texas, to the Uinta basin of Utah, and north through the northern Great Plains, crossing the border into Canada (Fig. 1). Declining natural gas production, higher gas prices, and new recovery technologies have focused interest on these resources.In the late 1960's and early 1970's, three attempts were made to stimulate tight formations by nuclear stimulation. Disappointing results ended the nuclear tests.In 1972, massive hydraulic fracturing (MHF) using volumes of fluids and proppants an order of magnitude or more larger than conventional was proposed. As a consequence, a joint government/industry test of MHF was conducted in the Piceance basin in 1974. The results of this test demonstrated that improvements could be achieved in the rate of gas flow. Since that time, MHF technology has been used widely in low-permeability but otherwise geologically favorable settings such as the Wattenberg field (north of Denver), the San Juan basin of New Mexico, and the Cotton Valley trend in east Texas and northern Louisiana.Despite the work to date, challenges posed by difficult geological settings - especially of the deep, tight, lenticular gas pays - have yet to be overcome. Even in the more geologically favorable basins - in the higher permeability and blanket type deposits fundamental improvements in recovery technology need to be pursued. Purpose and Methods This paper examines how improved technology and economics together can accelerate the contribution of the tight gas formations to U.S. energy supply. Overall results are presented for 13 tight gas basins for two levels of technology and three levels of economic incentives (represented by gas prices, although other incentives could be substituted). This is followed by a sensitivity analysis of specific technology improvements. JPT P. 1545^

Modeling of Hydraulically Fractured Gas Wells Completed in Noncontinuous Lenticular Formations

A mathematical model is presented which forms the basis for a reservoir simulator that can be used to assist in the interpretation and prediction of the performance of hydraulically fractured gas wells completed in noncontinuous sand lense formations. Equations are derived to simulate both reservoir and well performance. They are general in that the following effects are considered: 1) noncontinuous sand lenses, 2) three-dimensional flow in the reservoir and one-dimensional flow in the fracture, 3) non-Darcy flow in the reservoir and fracture, 4) wellbore and fracture storage, 5) formation damage on the fracture face, 6) frictional pressure drop in the production string, and 7) Klinkenberg effect. An operational two-dimensional gas reservoir simulator has been developed which includes these effects.

Tertiary Lacustrine Sediments from Sentinel Butte, North Dakota and the Sedimentary Record of Ectogenic Meromixis

ABSTRACT About 10 meters of probable Oligocene or Miocene lacustrine sediments which crop out on top of Sentinel Butte in western North Dakota consist mostly of greenish-gray zeolitic mudstone interbedded with relatively thin layers of dolomite, limestone, and chalcedony. The dolomite layers are several centimeters thick and are mostly dolomicrite, with laminae or lenses of arkosic sand and pellets. They probably formed by deposition of protodolomite in shallow, closed, saline lakes. The limestone layers are up to 30 centimeters thick. Their basal portions contain varve couplets of micrite and organic matter. Graded bedding in micrite laminae and concentration of aeolian silt in organic laminae suggest that deposition of carbonate took place during a relatively brief portion of each year. Pres rvation of varves, fossil fish, and coprolites in the limestone imply deposition in a meromictic lake. In contrast, poor preservation of fish and bioturbation in the upper parts of two limestone beds record transition to holomictic conditions. Some variations in varve thickness appear rhythmic and possibly related to the sunspot cycle. Several asymmetric cycles of contrasting carbonate facies, ideally consisting of massive dolomite overlying clay and underlying laminated limestone, imply that the ancient lake in which the sequence was deposited underwent several episodes of shrinkage and closure during which its salinity increased, followed by expansion accompanied by abrupt development of ectogenic meromixis. Changes in authigenic mineralogy and upward increase in thickness of limestone beds relative to dolomite beds record a long-term trend toward freshening of basinal hydrology. Models involving ectogenic meromixis can explain intimate associations of evaporitic facies with varved sediments in other sedimentary sequences and may be applicable to some Precambrian iron formations.

Progradational Sequences in Miocene Shoreline Deposits, Southeastern Caliente Range, California

ABSTRACT An exceptionally well exposed marine-nonmarine transition in middle Miocene strata exists in the southeastern Caliente Range, California. About 50 individual progradational sequences form a succession that ranges in thickness from approximately 1000 m (where predominantly nonmarine) to more than 2500 m (where predominantly marine). Paleogeographic evidence in basalt flows near the top of the succession and in overlying fluvial deposits indicates that these middle Miocene strata were deposited across a north-northwest trending shoreline. A complete progradational sequence typically is several meters to a few tens of meters thick and includes strata that represent three intertonguing stratigraphic units. Individual sequences generally rest on a thin gravel deposit interpreted as a transgressive lag on an erosional surface. The gravel is overlain by structureless siltstone or fine-grained sandstone deposited at water depths where the rate of faunal mixing exceeded that of production of structures by physical processes. These rocks grade upward into bedded fine sandstone deposited closer to shore where physical processes exceeded bioturbation. Crossbedded lenses of coarse sand or fine gravel in the upper part of this facies suggest the presence of fairly long-period surface waves. The bedded fine sandstone is sharply ove lain by a cross bedded coarse sandstone facies that is interpreted as a combined offshore bar-rip channel-surf zone assemblage. Cross-strata dip dominantly offshore, suggesting substantial deposition from rip currents. A secondary, shoreparallel mode of cross-strata direction suggests longshore currents produced by surface waves from the northwest. The crossbedded coarse-grained sandstone grades upward into planar-bedded medium-grained sandstone that is interpreted as a beach foreshore. This facies grades upward through structureless medium-grained sandstone into nonmarine or lagoonal red and green mudstone of the Caliente Formation. The middle Miocene succession was deposited in a subsiding basin that was otherwise remarkably stable tectonically; the position of the strand line differed no more than a few kilometers through a period of 1 to 3 m.y. The average duration of the transgressive-regressive cycles, a few tens of thousands of years, together with their distribution in groups of three or four in the lower two-thirds of the succession, is consistent with the pattern of long-term climatic cycles produced by periodicity of the earth's solar orbit and may be related to eustatic sea level changes attendant to the development of the Antarctic ice cap. Changes in the pattern of progradation in the upper part of the succession and nearby basaltic eruptions may have been precursors to the onset of movement along th San Andreas fault in this area 12-14 m.y. ago.

Variability of Sedimentary Textures and Processes on Continental Margin North of Wilmington Canyon: ABSTRACT

Sedimentary properties and processes of a 7,500-km2 corridor seaward of the Baltimore Canyon Trough off New Jersey were studied in detail using over 100 bottom samples consisting of grab samples and box, hydroplastic, and piston cores. The sediments in both canyon and intercanyon areas are primarily bioturbated, olive-gray, sandy silts with local features indicative of gravity-induced mass sediment movements (i.e., graded sequences and load structures). C14-based sediment accumulation rates vary by a factor of three in cores separated by as little as 6 km. These variations seem to be a function of shelf-edge spillover rates. Detailed analyses of the sand grain-size distribution throughout the corridor reveal that the sand component of the slope and rise sediments contains a high percentage of material currently present on the adjacent shelf. The relatively large percentage of sandy sediment in the upper parts of cores from Spencer Canyon suggests its recent role in transporting shelf sediments seaward. Sandy intervals in other slope cores are commonly obscured by intense bioturbation. Cores from the continental rise show an upward decrease in the number of sand layers and lenses. Active transport of sediment to the slope and rise occurred during the late Pleistocene and, although the intensity has declined, the slope is presently the site of deposition for both fine and coarse sediment. End_of_Article - Last_Page 1662------------

Evaluation of Injection Well Stimulation as Applied to a Large Micellar/Polymer Project

This paper deals with completion techniques applied to a large (407-acre or 1.6 km2) micellar/polymer flooding project. Injectivity and vertical conformance in wells completed openhole by solid-block shooting and hydraulic sand fracturing are compared. Openhole hydraulic sand fracturing appears to give better injectivity and vertical conformance than openhole shooting. Introduction Marathon Oil Co. currently operates a large micellar/polymer project in southern Illinois. This commercial-scale test, known as the M-1 Maraflood (trade mark) Project, lies in Secs. 15, 16, 21, and 22, T6N, R13W, Crawford County, IL, and encompasses approximately 407 acres (165 ha) of the main Robinson sand reservoir (Fig. 1). This Pennsylvanian-age sand is an example of stream channel deposition from a meandering river with migrating point bars. The reservoir lies about 950 (290 m) subsurface, and sand thickness varies from zero on the eastern flank of the project to about 60 ft (18.3 m) near the center (Fig. 2). Both multiple sand bodies stacked one above the other and isolated sand lenses found above and/or below the main sand occur in the M-1 project. Average reservoir characteristics are shown in Table 1. The Robinson sand reservoir was discovered in 1906 and experienced rapid and extensive development. Production during the first few years declined as the solution gas energy dissipated. Early attempts to reduce production decline consisted of air and gas injection and application of vacuum to producing wells. The project area subsequently was producing wells. The project area subsequently was developed for waterflooding on 10-acre (4-ha) spacing during the 1950's. Following a successful waterflood, oil production rates declined to a low level. In 1974, Maraflood development began. As indicated in Fig. 2, the project has been developed as two distinct areas; one on 2.5-acre (1.0-ha) spacing and the other on 5.0-acre (2.0-ha) spacing. Project development, including all wells and facilities, neared completion on Sept. 30, 1976, when a contract was executed between the U.S. Energy Research and Development Admin. (now DOE) and Marathon for cooperative funding of the project. This paper presents the history of injectivity problems, presents the history of injectivity problems, subsequent stimulation, and evaluation. Past experience has shown that injection of micellar solution is difficult unless the wellbore is undamaged and the micellar solution has been filtered and delivered to the wellbore uncontaminated. Analysis of injectivity problems requires a basic understanding of the recovery process. For the M-1 project, micellar/polymer process. For the M-1 project, micellar/polymer flooding is referred to as the Maraflood oil recovery process when used as described in Marathon's more process when used as described in Marathon's more than 100 patents covering the process. It typically consists of injecting between 5 and 15% PV of a micellar solution that displaces oil in a miscible-like manner and leaves a very low residual oil saturation. Micellar solution is displaced by a mobility buffer, normally a polymer-thickened water, to ensure adequate mobility control and provide good sweep efficiency. Finally, the mobility buffer is followed by drive water to complete the project. JPT P. 777

Eocene tidal sedimentation in the Anambra basin, Southern Nigeria

The Eocene Nanka Formation of the Anambra basin in southern Nigeria consists, in its type area, of four sand subunits each 50–90 m thick, separated by three gypsiferous glauconitic shales each about 2.5 m thick. The sand subunits are unconsolidated, uncemented quartzarenites, planar and trough cross-stratified, flaser and lenticular bedded, and burrowed ( Ophiomorpha and Skolithos). Texturally the sands are medium to coarse, moderately sorted, positively skewed and leptokurtic. Two parallel belts, each with a distinctive paleocurrent pattern, are identified: one lying along the present eastern and northern margin of the formation, characterised by a shore-normal paleocurrent pattern; and the other lying on the west, distinguished by a shore-parallel orientation. An integrated analysis of stratigraphic, petrographic, textural, and structural data of the formation enabled reconstruction of an environmental model for the sand body. The model depicts sedimentation in a tidally influenced marine shoreline environment in which an intertidal and a subtidal zone can be delineated. Each zone is characterised by an assemblage of several sedimentary features. Interlaminations of shale and thin sand lenses, gypsiferous and glauconitic shale beds, flaser and lenticular bedding, herringbone structures, and shore-normal paleocurrent pattern are among the features that delineate the intertidal facies. The abundantly cross-stratified, coarse elastic sand-bars with rapid fining-upward sequences, burrowed pebbly horizons overlain by fine sand, and shoreparallel paleocurrent are the distinguishing features of the subtidal facies. The sandflat facies of the intertidal zone and the sandbar facies of the subtidal zone are, however, inseparable in most cases.

Opal-Cristobalite-Cemented Sands in Catahoula Formation--Implications on Source of Silica Cementation of Quartzose Sandstones: ABSTRACT

Channel sands and sand lenses in the Catahoula Formation, an Oligocene-Miocene fluvial unit of the Gulf Coast, are in places cemented by an opaline material. Petrographic, X-ray diffraction, and electron microscopy studies indicate that the cement (1) shows poorly developed banding of varying birefringence (or degree of crystallinity); (2) has shrinkage cracks apparently resulting from ordering and dehydration; and (3) consists of opal and crypto- to micro-crystalline cristobalite which occurs as lephispheres about 2 µm in diameter. The cement developes in sands that are encased in tuffaceous silty mudstones. The development is apparently restricted to outcrops and near-surface zones of high permeability and appears to be pedogenic in origin. Pervasive alteration of the rhyolitic tuffaceous mudstones to clay minerals gives rise to excessive free silica, which is carried by groundwater to the permeable sandy zones. The silica was initially precipitated in sand interstices as a silica gel. Subsequent dehydration and ordering produces the opal-cristobalite observed. Further crystallization of opal-cristobalite during burial diagenesis may result in chalcedony or quartz cements. Calculations show that for a rhyolite ash to alter completely to 2:1 layer clay minerals (smectites), as much as 35 wt. % of the ash may be released as hydrogen silicate. This suggests that ash beds are significant sources of silica cement in sandstones. End_of_Article - Last_Page 738------------

Early Diagenetic Glauconitization of Feldspar Grains in a Middle Proterozoic Tidal-Flat Complex: ABSTRACT

Extensive glauconitization of feldspar grains exists in arkosic sedimentary rocks of a middle Proterozoic carbonate tidal-flat sequence in the Godavari valley, south India. Glauconitized sedimentary rocks occur as (1) well-defined thick layers, (2) small lenses at different stratigraphic levels, and (3) infilling in the intercolumnar areas of SH-stromatolite structures, in colonies of SS-type structures, and shrinkage cracks in algal mats. Glauconite replaces potash feldspar and plagioclase along cleavage and fractures. Varying degrees of replacement from incipient to complete are observed. Petrographic studies reveal two phases of glauconitization. The major phase, affecting detrital grains, is early diagenetic and is almost syndepositional. A second phase, affecting authigenic feldspar overgrowth, follows silicification. Primary structures and sediment characters of glauconitized sandstones indicate that glauconite formed in varying conditions of turbulence and bathymetry. A glauconitic layer in Ramgundam (18°45^primeN, 79°26^primeE) appears to have formed in a quiet environment, at moderate depth below wave base, during brief periods of transgression. Large bed forms in a thick layer near Manthani (18°33^primeN, 79°40^primeE), indicate a turbulent environment. Small sand lenses within carbonate rocks reflect rapid deposition by high-velocity currents. Close association of glauconite with SH and SS-type stromatolites indicates origin in agitated environments at very shallow depth, and supratidal origin is implied by its presence in shrinkage cracks within algal mats. Evidence of organic activity to aid glauconitization is absent. Even if blue-green algae might have effected glauconitization of detritus associated with stromatolites by providing small areas of reducing environment within a strongly oxidizing macroenvironment, glauconitization in the thick, sandy zones appears to be independent of biochemical processes. End_of_Article - Last_Page 691------------

Kurten Field--Discovered by Stratigraphic Prospecting: ABSTRACT

The Woodbine Kurten field trap is a stratigraphic sand lens surrounded by shale. This field is a significant discovery in Brazos, Grimes, and Madison Counties, Texas. Structure plays only a minor role in oil entrapment. The sand was deposited in a structural and topographic low demonstrated by comparing maximum pay sand thickness with a computer trend surface residual thick derived from Austin to Buda isopach map. Sand migration southward down the East Texas trough was deflected westward by the Angelina-Caldwell flexure and caught in a sag at the Kurten field locality. The higher Madisonville nose to the northeast has essentially no sand at the crest. Structural elevation at Madisonville and on the Angelina flexure only serves to limit sand deposition. Many porosity pod-type fields may result by looking for sags near a sand-shale regional boundary. End_of_Article - Last_Page 1566------------

Operational Problems in North Burbank Unit Surfactant/Polymer Project

The North Burbank pilot project is being conducted to determine the feasibility of a surfactant/polymer project that involves injecting a series of chemical solutions: freshwater preflush, salt-water preflush, surfactant, and polymer. This paper discusses operational problems encountered in each stage and describes alternative approaches used to minimize adverse effects on overall performance. Introduction In 1975, Phillips Petroleum Co. initiated a pilot project to demonstrate the feasibility of applying a surfactant/ polymer flood to an oil-wet sandstone reservoir that was polymer flood to an oil-wet sandstone reservoir that was nearing the end of its producing life. This paper describes, in order of the chemical injection phases, the chemicals used in the surfactant/polymer process, the mechanical system constructed to mix and inject the chemicals, the operating procedures developed, and the problems encountered. The North Burbank Unit (Fig. 1), located in Osage County, OK, has been producing under waterflood since 1950. Discovered in 1920, the unit produces a sweet 39 degrees API (828-kg/m3) gravity oil from the Burbank sand-stone, a well-consolidated sand lens averaging 47 ft (14.3 m) thick at depths of 2,700 to 2,900 ft (823 to 884 m) and covering about 24,500 acres (9914.8 m2). As of Aug. 1979, the North Burbank Unit (NBU) had yielded 299,400,000 bbl (476 Mm 3) of oil, through primary and secondary recovery, out of an estimated 671,300,000 bbl (1070 Mm3) of oil originally in place, Oil production has declined to the point that the unit is classified as stripper production. As operator of NBU and under contract to the production. As operator of NBU and under contract to the U.S. ERDA (now DOE), Phillips developed a 90-acre (36.4-m2) pilot project to test the feasibility of applying a pilot project to test the feasibility of applying a surfactant/polymer flood to the unit to extend the producing life and increase ultimate oil recovery. producing life and increase ultimate oil recovery. General Field Operations: NBU Tract 97 The pilot project being conducted at NBU Tract 97 (Fig. 2) has been operating since Oct. 1975. The well pattern consists of nine 10-acre (4.05-m2) inverted five spots with an injection well surrounded by four producing wells. In the waterflood operations carried on at NBU, the oil wells produce at an average water/oil ratio of 97. The produced produce at an average water/oil ratio of 97. The produced fluid is transferred from the wells through a gathering system to one of several tank batteries for separation and LACT sales. A small amount of fresh water is mixed with the produced salt water to make up the injection volume produced salt water to make up the injection volume requirements and is injected back into the Burbank sand. In the pilot project being conducted at NBU Tract 97, the injection fluid for a period of 27 months consisted of a specially designed series of chemical solutions (Fig. 3).The production and injection fluids for NBU Tract 97 are handled at a tank battery (Fig. 4) and a water station (Fig. 5) located at Tract 97. These facilities were redesigned so that the production and injection fluids for Tract 97 would be isolated from the fluids for the other tracts.To form the inverted five-spot well pattern, 11 new producers and nine new injectors were drilled in 1975. producers and nine new injectors were drilled in 1975. The new producers have 7-in. (177.8-mm) OD casing set into the top of the Burbank and were completed with 2 1/2-in. (63.5-mm) sucker rod pumps and 3 1/2-in. (88. 9mm) OD EUE tubing set at 2,600 ft (792,5-m). The old producers used multiple casing strings with 5 1/2-in. producers used multiple casing strings with 5 1/2-in. (139.7-mm) OD liners set into the top of the Burbank. All producers were completed as open holes. JPT P. 11

Structures and textures in till indicating subglacial deposition

Five structural and textural features are discussed: (1) small lenses of sorted material, (2) smudges, (3) small‐scale deformations of till matrix and smudges by clasts, (4) clasts consistently striated, and (5) clasts with stoss‐and‐lee sides. Analyses suggest that these features may be produced by subglacial processes acting in the ice‐bed interface. Long axes of small sand lenses and smudges as well as the striation on the upper surface of scattered clasts in lodgement till have a strong preferred orientation in good agreement with the glacier flow direction as indicated by clast fabrics, bedrock striation, and surface fluting of ground moraine. When in traction against the till bed, clasts may plough up till banks. Clasts with stoss‐and‐lee sides development were also very distinctly oriented as their stoss sides faced significantly up‐glacier.It is concluded (1) that each of the five features discussed is useful as a criterion for subglacial deposition by lodgement, (2) that they indicate important differential movement along the ice‐bed interface and therefore suggest a temperate regime in this part of the glacier during the till deposition, (3) that very few orientation measurements of one or more of these features signify the ice movement direction; i.e. a time‐saving method to find the paleoflow direction of Pleistocene glaciers, and (4) that taken together with till preconsolidation, mechanical composition, and clast fabric, they may support each other and give good indications of the genesis of Pleistocene tills.

Volcanic Sediment Apron in the Tascotal Formation (Oligocene?), Trans-Pecos Texas

ABSTRACT A volcanic sediment apron in the Tascotal Formation consists mostly of sand-sized debris derived from nearby volcanic centers. In the apron, a progradational sequence of sheetflood deposits gives way upward to aggradational deposits formed as major feeder channels were gradually abandoned. Sheetflood deposits, here termed thin fining-upward sequences, begin with a lens of structureless or trough cross-bedded medium sand to granular coarse sand, which overlies a scour surface. The basal lens grades up into thin-bedded or structureless fine sand. Bedding planes in the upper part of the sequence are marked by evidence of episodic deposition such as sun-cracked mud drapes or burrowed, root marked or tracked horizons. Aggradational channel deposits, or thick fining-upward sequences, begin ith a thin--one or two clast thick--cobble or boulder conglomerate overlain by fining-upward subsequences that include tabular crossbeds, horizontal lamination, low-angle trough cross-bedded sandstone and mud-rich units. Volcanic activity contemporaneous with formation of the Tascotal apron was characteristically explosive, producing abundant pyroclastic material in addition to lava flows. A cover of fine debris sufficient to prevent erosion of lavas and ash flow tuffs that would contribute coarser material to the apron was maintained over the volcano slopes and adjacent uplands. With cessation of the activity of source volcanos, erosion quickly stripped this fine cover, and began to erode the flow rocks. With this, gravel replaced sand as the predominant sediment material. This change of grain size can be attributed to the cessation of volcanic activity rather than tectonic movements or climatic change.

Role of Temperature and Burial Depth in Development of Subnormal and Abnormal Pressures in Gas Reservoirs: ABSTRACT

The aquathermal-pressuring concept shows that isolated, water-filled reservoirs become abnormally pressured when temperature rises owing to increasing depth of burial. When reservoirs contain free gas, the situation is more complex and abnormal or subnormal pressures may develop depending on the gas/water ratio and the End_Page 414------------------------------ initial and final depth of burial. Because zones that have nonhydrostatic pressures must be effectively isolated from their surroundings (or the pressure should be equalized), a model would be a sand lens encased in shale. Consider, for example, where isolation occurs at 4,000 ft (1,200 m) where temperature is 123°F (51°C), and subsequent burial moves it to 8,000 ft (2,400 m) at 178°F (81°C). The pressure in the will start at 1,860 psi (12,815 kPa) hydrostatic at 4,000 ft (1,200 m), but the temperature rise will increase it to 2,035 psi (14,021 kPa), at 8,000 ft (2,400 m) is 3,720 psi (25,630 kPa), so the reservoir will be 1,685 psi (11,610 kPa) underpressured. Real reservoirs contain both and water. Calculations show that for trapping at 4,000 ft ( ,200 m) followed by burial to 8,000 ft (2,400 m) the reservoir will show various amounts of underpressuring if it contains more than 3 vol.% gas. With less than 3 vol.% it will overpressure. At greater trapping depths, high percentages of are needed to produce underpressuring, for example, 16 vol.% at 12,000 ft (3,600 m). Temperature decrease owing to uplift and removal of overburden produces the opposite effects, and reservoirs containing high percentages of develop abnormally high pressures. This theoretical model provides an explanation for the common occurrence of underpressured gas, particularly in stratigraphic traps with low water contents. It also explains the underpressured in the bottom of basins (such as San Juan, Wattenberg, and western Canada Deep Basin) and shows how abnormal and subnormal pressures can be developed in adjacent reservoirs in a restricted geographic area (such as the Appalachians). Regional tilting may bury a formation in one area but uplift it in another leading to regional trends from subnormal to abnormal pressures. An example of this is provided by the gas sands of the Morrow in western Oklahoma. End_of_Article - Last_Page 415------------

Hydrocarbon Occurrence as Function of Thermal Alteration of Organic Material: ABSTRACT

Fourteen wells from the Lenora gas field, Dewey County, Oklahoma, have been studied by vitrinite reflectance microscopy to determine maximum paleotemperature and temperature gradients. Various types of petroleum hydrocarbons (oil, distillate, and gas) are formed at varying temperatures which have been empirically related to the degree of vitrinite reflectance (Ro). Ro values at the depth of petroleum accumulation are consistent with the types of hydrocarbons encountered. Geochemical data obtained from well cuttings indicate that the petroleum originated in surrounding shales. Therefore, the Ro values obtained reflect accurately the maximum temperature to which the petroleum and its precursors were subjected. Reflectance gradients calculated for each well by taking Ro measurements at several depths in each borehole reveal a gradient anomaly directly over the reservoir when compared to the gradients existing beside the reservoir. The reservoir itself is a small sand lens, possibly of barrier-island or bar origin. It is possible, then, that determination of paleotemperature gradients by vitrinite microscopy and the identification of gradient anomalies in a basin may be useful in the search for new reservoirs. End_of_Article - Last_Page 544------------