Shale Interbeds Research Articles

Tectonic evolution of the Tertiary basin at Batu Arang, Selangor Darul Ehsan, Peninsular Malaysia

In the Batu Arang area are found Tertiary sedimentary rocks that outcrop within an approximately elliptical basin surrounded by undulating to hilly terrain developed over Permian meta­ sedimentary rocks consisting of quartzites, schistose quartzites, phyllites, indurated shales and rare cherts. The Tertiary strata cover an area of some 15 km2 and rest unconformably over the meta­ sedimentary rocks that generally strike NNW-SSE with variable, steep to vertical dips. The Tertiary strata can be separated into two sequences: an upper one termed the Boulder Beds that consists of semi-consolidated, sandy to gravelly and bouldery sediments that unconformably overlie the lower sequence known as the Coal Measures or Batu Arang Beds comprising interbedded shales, structureless clays, fine grained to pebbly sandstones and some lignite seams. The Boulder Beds consist mainly of sub-angular to rounded, pebble to boulder sized clasts of quartzite, sandstone and vein quartz as well as minor chert, schist, phyllite and shale fragments, in a matrix of gravel, sand or clayey sand, though a few beds of inter-stratified sand and sandy clay are also seen. Differences in colour, as well as differences in composition and roundness of clasts, has allowed for differentiation of three separate sequences within the Boulder Beds. The Beds are up to 300 m thick at the centre of the basin where their bedding is horizontal, though along the basin edges, they show inward dips of 20° to 45°. A number of small, syn-depositional, normal faults, and post-depositional, wrench faults, cut these beds whose age is not know, though a late Middle Miocene to Pliocene age is proposed by correlation with regional tectonic events. The Boulder Beds are alluvial fan deposits that have developed through erosion of steep, fault induced slopes. The Coal Measures or Batu Arang Beds consist of several facies arranged in the following stratigraphical order: i) Sandstone with shale and clay intercalations, ii) Shale, iii) Coal beds, comprising two main coal seams, shale and sandstone, and iv) Sandstone with shale and clay and minor conglomerates including a basal pebbly sandstone. The strata show a synclinal structure plunging gently southeastwards, with the strata having an average dip of 14° towards the centre of the basin, though local variations are common especially towards the basin edges. The fine grained sediments range from stiff, structureless clays to well laminated and fissile shales containing abundant carbonaceous materiaL The shales are also often silty and even sandy and grade into clayey sandstone in places. The inter-bedded sandstones are mostly fine grained, though ranging up to coarse grained, and even pebbly. The coal occurs mostly as thin laminae or streaks in shales, but also builds thicker layers of more than 30 cm thick. In the eastern part of the basin, two thick coal seams are found as well as some thin beds of coal and coaly shale. The two seams are about 65 m stratigraphically apart; the Upper Seam attaining a thickness of up to 15 m, and the Lower Seam averaging 8 m in thickness. The coal appears to be intermediate between high-grade lignite and sub-bituminous coal and is a hard, black rock with a resinous lustre. Palynomorph assemblages indicate an Eocene to Oligocene age for the coals that are likely to have been peat-swamp deposits in an alluvial plain setting, whilst the inter-bedded shales were deposited in a freshwater lacustrine setting. Correlation with known stratigraphic successions in Sumatra shows that the Coal Measures are time-equivalent (Late Eocene-Early Oligocene) with, and have a somewhat similar depositional history as, the Parapat Formation of the North Sumatra Basin, the Permatang Group and Kelesa Formation of the Central Sumatra Basin, and the Lahat Formation of the South Sumatra Basin. Differences in lithology, however, occur and reflect differences in geographical locations; the sediments at Batu Arang having been deposited at a higher altitude, and in a more inland location, as compared with the Sumatran Basins that formed at lower altitudes, close to the then Sundaland coastline. Differences in geographical locations have also influenced the subsequent geological histories of the individual basins. No lithological equivalent of the Boulder Beds is found in the Tertiary strata of the Sumatran Basins, though its deposition is considered to be time-equivalent (Middle Miocene-Pliocene) with the Baong Formation of the North Sumatra Basin, the Minas Group and Binio Formation of the Central Sumatra Basin, and the Air Benakat Formation of the South Sumatra Basin. Correlation with regional tectonic events indicates that a pull-apart basin (of half-graben shape)

Utility of Ground-Penetrating Radar in Near-Surface, High-Resolution Imaging of Lansing-Kansas City (Pennsylvanian) Limestone Reservoir Analogs

High-resolution ground-penetrating radar (GPR) is a subsurface imaging tool that can extend results gained from studies of reservoir-analog outcrops and add detailed information about reservoir analogs that is unavailable from either seismic data or well control alone. Integration of GPR-reflection information and outcrop photomosaics allowed detailed study of subtle changes in lithology and bedding surfaces by comparing lateral and vertical changes in GPR-reflection character with outcrop features. Outcrops are valuable for confirming interpretations of reflections and providing velocity information for the GPR data. Outcrops of two Lansing-Kansas City Group limestone units, the Captain Creek Limestone and the Plattsburg Limestone, were used as test sites to determine the vertical imaging resolution, penetration depth, and reflection character of high-frequency (500 MHz) GPR in interbedded carbonate and shale units, where the carbonate units contained thin interbeds of shale. Features as small as 0.1-0.2 m (0.3-0.7 ft)--including major architectural elements (such as major and minor bounding surfaces) and internal features (such as fractures, internal bedding, and crossbedding)--were successfully imaged and confirmed by outcrop data. Variations in GPR-reflection character between geologic units allowed recognition of argillaceous limestone units in the subsurface. Although shale and soil at the surface generally impeded GPR signal penetration, thin shale layers and shale at bounding surfaces actually enhanced reflectivity and aided in interpretation. Our results indicate that GPR can be successfully used as an aid in outcrop studies to provide quantitative data for use in reservoir modeling.

An Assessment of In Situ Oil Sands Recovery Processes

Development of Alberta's 1.7 trillion barrels of oil sands resources has been growing rapidly. Significant factors in this growth include conventional oil production declines, oil price and spread improvements, increasing pipeline access to markets, a favorable fiscal regime and imminent lease expires. The recent successes with in situ processes including cold flow, cyclic steam stimulation (CSS) and steam-assisted gravity drainage (SAGD) have increased in situ production from Alberta oil sands by 65 % in just two years to over 34,000 m3/d. The growing successes of SAGD and cold flow have created an energy to implement these processes that is reminiscent of CSS in the late 70s and early 80s. Oil sands reservoirs are complex. Coupled with high bitumen viscosities, a commercial development must address a range of reservoir properties including variable bitumen saturations, ranges of reservoir thickness and sand qualities, the presence of shale interbeds and diagenetic clays and the impacts of gas and water thief zones. In the process of evaluating the three aforementioned processes it is important to gauge the type and amount of resource that each can develop. The purpose of this overview is to summarize key success factors and the applicabilities of the CSS, SAGD and cold flow processes at accessing the bitumen resource. Before providing the comparison, a background understanding of the evolution of Cold Lake CSS and the mechanisms controlling its performance are provided. Cold Lake Technology Development Imperial's Cold Lake operations are an example of successful in situ oil sands technology development. When Imperial started at Cold Lake in 1964, there was no recovery process and no assurance that economic development was possible. As shown in Figure 1, it took approximately 20 years of piloting a number of processes before commercial production could occur. Since 1985, volumes have grown to the current 100,000 b/d. Over time, many processes were examined and even field tested. The most successful has been CSS. However, before describing CSS further, it is worth mentioning a few of the other field tested processes. The most obvious was steam drive. In the 60s steam drive was being successfully developed in California so that it appeared to be just a matter of translating the California successes to Cold Lake. This meant that CSS would simply be a precursor to steam drive. The initial purpose of CSS was thus to obtain early production and develop communication between the CSS wells. Continuous steam drive processes of various types were initiated in the early stages, at mid-life and during late stages of CSS. None were commercially feasible. A second alternative to CSS, SAGD, was invented and patented. Testing started in 1979. Disappointed again with the constant pressure performance, the horizontal well pilots now operate in a cyclic steam stimulation mode. Another process, Hydraulic Borehole Mining, to jet the sands and produce a slurry to surface proved technically impossible due to the collapse of bounding shales. Finally, cold flow was attempted but rates were low and unsustainable.

Tectono-eustatic controls on reservoir compartmentalisation and quality — an example from the Upper Miocene of the San Joaquin basin, California

Systematic changes in stratigraphic architecture and reservoir quality of Upper Miocene Stevens sandstones at North and South Coles Levee fields, San Joaquin basin reflect tectono-eustatic controls on deposition. Two depositional sequences (330–500 m thick) are present in the Coles Levee Stevens. The upper sequence contains sandstones deposited by sediment-gravity flows in water depths of 300–500 m. Backstepping (retrogradational) reservoir compartments in this sequence overlie vertically stacked to prograding compartments. Although the backsteps are the better reservoirs with higher permeabilities and porosities, the vertically stacked to prograding compartments are volumetrically more important. Curiously, the upper Stevens section is sand-rich (60–80% ss) on structural highs that were also palaeotopographic highs. Thicker sections with less and characterise the flanks. These sand distributions are anomalous since sediment-gravity flows preferentially deposit in bathymetric lows. Apparently, the flanks of Stevens palaeohighs subsided faster than the crests. Consequently, accomodation increases were greatest on the flanks and resulted in aggradation with preservation of section. Less accommodation on the crests resulted in progradation with erosion and reworking. Through lobe-switching processes some sand reached all parts of the depocenter. Subsequent progradation with erosion of shale interbeds and attendant reworking by density currents created sand-rich sections in slowly subsiding areas (crests). Concurrent aggradation preserved shale interbeds and created shale-dominated sections in rapidly subsiding areas (flanks). Greater compaction of thicker, shale-dominated flanks equalised thicknesses of flank and crest sections. Thus, the reservoir architecture represents the integrated effects of compensation (lobe-switching), accommodation (subsidence + sea level), and compaction.

Heterogeneous Distribution of Calcite Cement at the Outcrop Scale in Tertiary Sandstones, Northern Apennines, Italy

Calcite cement derived intraformationally in seven stratigraphic units of marine origin (five submarine-fan deposits and two shelf deposits) is distributed heterogeneously at the outcrop scale. Sandstone beds intercalated with calcareous shale older than Pliocene tend to be completely cemented, whereas stacked sandstone beds that lack shale interbeds have calcite cement in the form of tightly cemented concretions that make up only 10-30% of a bed. The abundance and distribution of concretions, with few exceptions, are irregular and unpredictable. Concretion shapes include spheres (<1 m diameter), oblate and prolate spheroids (<1.5 m), tabular forms (to 8 m long), and irregular forms. Patterns of concretions within beds are remarkably varied and include both random and uniform spacing; preference for either the top, middle, or bottom of beds; preference for faults that cut bedding at a high angle; and localization around shale rip-up clasts. There is no preference of concretions for shell-rich layers. Some formations have cement patterns specific to that formation, whereas other formations have different patterns at different outcrops. Most formations have more than one cement pattern in an outcrop. The lack of strong textural (grain size, graded bedding) or compositional controls on the localization of calcite cement suggests the preeminence of highly localized hydrologic factors in determining the spatial distribution of authigenic pore-filling calcite. Spherical concretions grew by diffusive supply of intraformationally derived components, whereas prolate and elongate concretions grew chiefly under the influence of advective supply. Faults apparently served as fluid conduits and were selectively cemented. In general, only sandstones intercalated with shale are totally cemented. This indicates that shales were a major source of cement components for these sandstones at least.

Neocomian Source and Reservoir Rocks in the Western Brooks Range and Arctic Slope, Alaska: ABSTRACT

Detailed (1:63,360) mapping of the Tingmerkpuk sandstone and associated rocks in the Misheguk Mountain and DeLong Mountains guadrangles of the western Brooks Range thrust belt documents potential hydrocarbon source and reservoir rocks in the northern foothills of the western Delong Mountains and adjacent Colville basin of northwest Alaska. Neocomian (?) to Albian micaceous shale, litharenite, and graywacke that overlies the Tingmerkpuk represents the onset of deposition of orogenic sediments derived from the Brooks Range to the south, and the merging of northern and southern sediment sources in the Colville basin. Both the proximal and distal Tingmerkpuk facies contain clay shale interbeds and overlie the Upper Jurassic to Neocomian Kingak Shale. Preliminary geochemical data show that in the thrust belt, these shales are thermally overmature (Ro 1.4-1.6), but are good source rocks with total organic content (TOC) that ranges from 1.2 to 1.8 percent. Shale in the overlying Brookian rocks is also thermally overmature (Ro 1.2-1.5 percent), but contains up to 1.8 percent TOC from a dominantly terrigenous source, and has generated gas. In outcrops at Surprise Creek, in the foothills north of the thrust belt, the Kingak (1.9 percent TOC) and underlying Triassic Shublik Formation (4.6 percent TOC) are excellentmore » oil source rocks with thermal maturity close to peak oil generation stage (Ro0.75-0.9 percent). These rocks have lower thermal maturity values than expected for their stratigraphic position within the deeper parts of the Colville basin and indicate anomalous burial and uplift history in parts of the basin. Preliminary apatite fission-track (AFTA) data from the thrust belt indicate a stage of rapid uplift and cooling at about 53.61 Ma.« less

Review of the biostratigraphy of the Aruma Group (Upper Cretaceous) in the Arabian Peninsula and surrounding regions

The Aruma Group (Coniacian-Maastrichtian) in the Arabian Peninsula and surrounding regions consists mainly of limestone with subordinate dolomite and shale interbeds. It is well exposed in central Arabia, but in the eastern regions it occurs only in the subsurface. In the outcrop it has been treated as a formation and divided into three members (Khanasir Limestone, Hajajah Limestone and Line Shale), and nine biostratigraphical assemblage zones (Tissotia, Lopha/Ostrea, Sphaerulites/Biradiolites, Cardium/Protocardium, Gryphaea, Cyclolites, Orbitoides/Lepidorbitoides, Spondylus/Cymatoceras and Omphalocyclus macroporous/Fissoelphidium operculiferum ). In the subsurface, it has been given a Group rank and in Kuwait it is divided into three formations (Gudair, Bahrah and Tayarat) and six biostratigraphical zones (Globotruncana renzi, G. elevata/G. coronata, G. stuarti stuartiformis, First zone with no plankton, G. gansseri, and Second zone with no plankton). In the subsurface of Qatar and Abu Dhabi, the group includes four formations for which different terminologies have been used.

Early proterozoic black shales of the Timeball Hill Formation, South Africa: volcanogenic and palaeoenvironmental influences

Black shales in the Early Proterozoic Timeball Hill Formation exhibit a widespread dark grey colour due to disseminated iron minerals, predominantly limonite after pyrite, with subordinate thin beds and laminae more intensely pigmented by finely disseminated flakes of carbonaceous material. Andesitic-basaltic volcanism is thought to have provided the source of iron and sulphur for the ferruginous colouration in a basal and uppermost black shale facies. Sulphate-reducing bacteria around volcanic vents possibly produced the organic matter for the darker beds. Turbiditic rhythmites and succeeding fluviodeltaic sandstones overlie the basal black shale facies and associated Bushy Bend lavas. In the south west of the basin both rhythmites and sandstones have black shales as either thin interbeds or matrix material suggesting the possibility of continued fumarolic emissions in this region. A repetition of the rhythmite facies again shows an association with subordinate thin black shale interbeds in the southwest of the basin and this facies is succeeded by a second occurrence of the black shale facies, underlying the Hekpoort Andesite Formation. The observed association of black shales with turbiditic rhythmites, lavas lacking pillow structures and fluviodeltaic sandstones suggests water depths which varied from shallow to a few hundreds of metres. Some potential for volcanogenic massive sulphide and sedimentary exhalative ore deposits exists in the black shale as there is volcanic rock associated at the base of the formation.

Unsaturated Transport Processes in Undisturbed Heterogeneous Porous Media: I. Inorganic Contaminants

AbstractProlonged disposal of organic and inorganic waste in shallow land burial sites throughout the USA has prompted detailed investigations of subsurface contaminant transport processes. The fate and transport of contaminant leakage from pits, trenches, and cribs into the vadose zone via storm events is not well understood. The objective of this study was to investigate the thermodynamic and kinetic processes controlling the transport of inorganic contaminants in unsaturated, heterogeneous subsurface media. Large undisturbed columns were isolated from a proposed waste site consisting of fractured saprolite (weathered interbedded shale and limestone), and steady‐state nonreactive and reactive solute transport experiments were performed at a variety of pressure heads. Observed breaktbrough curves (BTC) for binary and ternary mixtures in the Co‐Sr‐Ca system were delayed relative to nonreactive Br BTC, indicating that the former tracers were adsorbed by the solid phase. Transport of the binary mixtures Co‐Ca and Sr‐Ca was predicted reasonably well with the equilibrium convective‐dispersive (CD) equation using independent measurements of all model parameters. However, application of the nonequilibrium or kinetic CD model to the observed Sr and Co binary data resulted in an improved description of contaminant transport. Cation‐exchange equilibria relationships on homogenized subsurface material, using both shake batch and miscible displacement methods, adequately described the thermodynamic processes that were prevalent during contaminant transport. These results suggest that preferential transport of reactive contaminants is negligible for the unsaturated conditions used in this study, and that the structured saprolite within the subsurface media is a chemically active constituent during reactive solute transport. Although the transport of contaminants in the ternary Co‐Sr‐Ca system exhibited many of the same features as the binary transport studies, an attempt to simulate the transport of contaminants in the ternary system as independent species was less than adequate.

Misoa Formation Sandstone Diagenesis, Blocks III and V, Maracaibo Basin, Venezuela

The petroliferous Lower-Middle Eocene Misoa Formation of the central Maracaibo Basin, Venezuela, is considered an ideal rock unit for investigation of controls on reservoir diagenesis. The Misoa Formation of the central Basin is typically a fine to medium grained, moderately to well sorted sandstone, with shale interbeds, representing deposition in various delta front to shallow marine environments. Sandstones are mainly sublitharenites and quartzarenites. The diagenetic evolution of the Misoa Formation in Blocks III and V of the central Basin area has been established through quantitative basin modeling, and detailed petrographic analysis involving thin section study, scanning electron microscopy, and X-ray diffraction analysis. The reservoir sandstones have been substantially modified by diagenesis. Porosity enhancement has been created in these rocks through the dissolution of unstable framework grains and carbonate cements. A major control on dissolution may be organic acid flushing prior to hydrocarbon generation; a mechanism supported by quantitative basin modeling results. A second control on porosity enhancement and reservoir quality is extensive leaching by meteoric waters near the post-Eocene unconformity surface. Sandstones nearer this surface have excellent porosity and permeability, and display profuse leaching of grains, clays, carbonate cements, and even quartz. Although Misoa sediments were affected by basin-wide tectonismmore » including initial Eocene subsidence, an extended period of post-Eocene uplift and erosion and Miocene subsidence, the paragenetic sequence appears uniform. This diagenetic scenario reflects early burial diagenesis, telediagenesis, and medium to late burial diagenesis stages. Authigenic minerals include various carbonate, silica, and clay phases. The clay mineral assemblage in this area is represented by common kaolinite, with subordinate discrete illite, and ordered mixed-layer and expandable mixed-layer illite/smectite, chlorite.« less

Eagle Lake Viking Miscible Displacement Laboratory Studies

Abstract A series of displacement tests was conducted on preserved reservoir core plugs to assist in estimating waterflood and miscible flood performance potential for the Eagle Lake Viking Reservoir in Saskatchewan. The reservoir rock is a heterogeneous sandstone, comprising thin interbeds of sand and shale with the sand lenses containing widely different amounts of smectite clays. To assess miscible recovery efficiencies in this complex sand, displacement tests were conducted with first-contact miscible propane and carbon dioxide solvents. Using standard waterflood relative permeabilities and a dual permeability model, all miscible displacements were matched by simulation with a Todd-Longstaff-type miscible mixing model. Introduction The Eagle Lake Viking Reservoir is located in southwest Saskatchewan as shown in Figure 1. It is a sandstone formation of Mississippian age. Discovered in 1957, one half of this 12.7 million m3 field was put on waterflood ten years later in 1967. Unfortunately, because of its low permeability, it has been possible to only inject 0.14 pore volumes of water in the subsequent 2O-year period. As a result, in the thirty years since discovery, only 15% of original oil- in-place has been produced. The cause of poor performance appears to be due to the abundant shales and days. Thin section analyses, however, show that bands of higher permeability are also found throughout the pay zone and often exist as thin to very thin sand stringers which are relatively free of days. In 1985, operations engineers decided that the half of the field which was still under primary depletion should be placed under some form of secondary recovery. Because the waterflood had been less than satisfactory, it was decided to examine miscible flooding as a potential alternative. Hence, a core flooding study using carbon dioxide as a flooding agent was designed with the following objectives;quantify carbon dioxide oil displacement efficiency;compare carbon dioxide with waterflood oil recovery; andcompare the potentially complex carbon dioxide miscible oil displacement process with a more conventional hydrocarbon solvent process using propane. To extend the initial objectives for reservoir design purposes, a fourth objective was to:obtain waterflood and miscible displacement parameters for use in numerical simulation of field applications, Reservoir Fluid and CO2 Miscibility Pressure The live oil used in this study was obtained by recombining separator gas and liquid to a bubble point pressure of 6509 kPa at 22 °C. Fluid densities were measured as a function of pressure at the reservoir temperatures of 22 °C and were matched with the Peng-Robinson(l) equation-of-state. The reference 2 characterization procedure was used and viscosity predictions were made with the Jossi, Stiel and Thodos correlation. Table 1 illustrates the accuracy or this prediction which involved proprietary correlations for heavy end parameters. This set of density and viscosity data was used in the simulation study. The oil composition is also shown in Table 1. To determine the first-contact miscibility pressure for CO2, oil and CO2 were blended in a visual cell at 22 °C. These results and the Peng-Robinson equation-of-state matches are presented in Table 2.

The Saint‐Daniel Melange: Evolution of an accretionary complex in the Dunnage Terrane of the Quebec Appalachians

The Saint‐Daniel Mélange is part of a series of mélanges located along the Baie Verte‐Brompton line in the Northern Appalachians. This line marks the suture between rocks of oceanic affinities and those of the ancient passive margin of North America with which they collided during the Taconian (Middle to Late Ordovician) orogeny. The Saint‐Daniel Mélange contains a wide variety of lithologies including well‐bedded to dismembered sedimentary sequences, pebbly mudstone, olistostromes, and slivers of igneous and metamorphic rocks. Black shales with interbeds of green shale, calcareous siltstone, or sandstone are the dominant units. They exhibit various stages of mélange formation such as those present in shallow parts of an accretionary complex. Units of oceanic origin include sediments derived from the forearc basin and slivers of an ophiolite and of a magmatic arc. Units derived from sediments of the passive margin of North America are also present. The ratio between these various lithologies changes greatly within the mélange on a kilometric scale along strike. The Saint‐Daniel Mélange is a structural complex in which the various units were assigned a sequential order mimicking a stratigraphic order. The Saint‐Daniel Mélange is interpreted as the relict of an accretionary complex because of its actual structural position within the Northern Appalachians and because all its lithologies and their structural fabric can be found in modern accretionary complexes.

Diagenetic formation of bedded chert: Evidence from chemistry of the chert-shale couplet

Theories concerning the formation of bedded chert traditionally have emphasized either depositional or diagenetic processes. Major and rare earth element data from Franciscan assemblage (Mesozoic) and Claremont Formation (Miocene) bedded chert sequences, along with physical observations such as the presence of rare and highly corroded radiolarians in shale interbeds, are most consistent with a dominantly diagenetic origin of chert-shale couplets and are incompatible with many depositional theories. Chemical distributions between Franciscan and Claremont bedded chert-shale closely match chemical fractionations recorded by Monterey Formation and Deep Sea Drilling Project-sampled cherts formed by diagenetic SiO{sub 2} dissolution, transport, and reprecipitation, suggesting that diagenetic migration of SiO{sub 2} from proto-shale to proto-chert is also largely responsible for chert-shale couplets. Identical Ce anomalies (Ce/Ce*) found in immediately adjacent chert-shale layers indicate that turbidites or other transport mechanisms are not responsible for the alternating beds. Neither the chemistry of the chert-shale couplet nor the overall stratigraphy of the sequences is consistent with couplet formation being caused by productivity fluctuations. Chemical mass balance calculations reconstructing the total bulk sediment composition suggest that modern siliceous sequences do not contain enough labile biogenic SiO{sub 2} to form entire stratigraphies of bedded chert.

Development of the Austin Chalk in Giddings Field, Burleson County, Texas, by Horizontal Drilling

ABSTRACT The Austin Chalk produces in a NE-SW trend, sub-parallel to the Texas Gulf Coast, in a band approximately 523 km. (325 mi.) long and 40-64 km. (25-40 mi.) wide. Giddings Field is located in the central part of this trend and covers a portion of several counties along the productive fairway. The Austin Chalk produces from a hard, dense, intensely fractured limestone with a matrix porosity range of 3-8% and permeabilities of less than 0.01 md. Production comes primarily from the fractures with contribution from the matrix coming later in the life of the well. The Austin Chalk was deposited near the southern end of the Cretaceous western interior seaway during the Coaniacian and Santonian ages (late Commachean to early Gulfian series) on a stable, gently dipping ramp during a high stand of sea level. The Austin was deposited as a calcareous ooze made up of coccoliths, coccolith fragments, planktonic foraminifera, and fine grained skeletal debris. The Austin is poorly to intensely burrowed and contains organic rich interbedded shale layers with gradational to sharp contacts with the intervening cleaner limestones. The high total organic content indicates the potential for the Austin Chalk to have been partially or completely self-sourced. As a result of the development and improvement of horizontal drilling techniques, the Austin Chalk has experienced a flurry in drilling activity. This boom began in Pearsall Field in the southern end of the trend and soon spread to the northeast to Giddings Field and beyond. Several different methods such as drilling a highly deviated hole or traverse across the Austin, drilling parallel to faulting, stair-stepping across different zones of interest, or drilling in a specific target for the entire well path have been tried (along with other ideas) with varying degrees of success. There are many different questions to be answered before drilling a horizontal well. Consideration should be given to off-set production, presence or absence of faulting, and thickness of the target zone to name just a few. The parameters and guidelines used in one area may not work the same way in a different area or in a different part of the same field. Seismic should be used in choosing locations and in evaluating areas of interest where there is limited subsurface control. For a typical horizontal chalk well, casing is set into the top of the Austin 6-9 meters (20-30 feet) at some predetermined angle which will allow for getting into the target zone as soon as possible. If needed, a tangent section is then drilled before continuing to build angle to reach the center of the target. The well is then drilled to the planned measured depth and completed. Early wells were cased but due to cost and the nature of the reservoir the subsequent wells were completed open hole. The early horizontal wells were drilled with lateral sections less than 609 meters (2000 feet). Improvements in drilling techniques have enabled operator to drill lateral sections greater than one mile. Both old and new methods are used for formation evaluation of the horizontal wells. Careful evaluation of the cuttings and close monitoring of the penetration rates and gases are critical in determining if the lateral section is being drilled in the right zone in identifying fractured intervals and in qualifying shows. New borehole imaging tools produce a visual record of the borehole and are good direct measurement indicators of fracturing along the well path. The data gathered from these logs can be used in many ways to aid in evaluation of the play. End_of_Record - Last_Page 801-------

Harriet Field – A Two Dimensional Modelling Study

A series of two dimensional interpolation models was constructed using wells from the Harriet Field. These models were designed to assess the effects of: (a) shale interbeds within the Flag Sand and (b) the fluid content (i.e. gas vs. oil vs. water) on the seismic response. The model responses closely resemble the seismic data. The results indicate that both of these factors have an impact on the seismic expression of the top Flag Sand interface and the associated amplitude anomaly which occurs immediately below it. In addition, apparent velocity variations which have been observed over the field are probably related, at least in part, to tuning effects resulting from shale and fluid content variations. In the models constructed, the presence of hydrocarbons correlates with low frequency zones in the seismic data which occur in association with high amplitude in the trough beneath the top Flag Sand reflector.

Depositional Environments and Geometry of Coal-bed Natural Gas Reservoirs in the Raton Basin, Colorado and New Mexico

Recognition of the depositional environments, occurrence, and geometry of Raton basin coals and coal-bearing rocks is essential to the reliable estimation of potential gas-in-place reserves and profitability, optimum placement and operation of production wells, and effective completion design for all Raton basin coalbed degasification projects. This knowledge is needed for two reasons: (1) coals of the Vermejo and Raton Formations, coalbed gas reservoirs in the Raton basin, are laterally discontinuous and separated by significant innerburden. (2) Vermejo and Raton Formation coals have been naturally coked or replaced by middle Tertiary igneous sills and dikes in many areas of the basin. Diagnostic criteria for facies recognition indicate the major environments of peat deposition included generally northeast-trending Vermejo interdistributary bays, northwest-trending Vermejo distributary-mouth bars, and northeast-trending Raton alluvial plains. Stratigraphic, sedimentologic, and coal petrographic data indicate Raton basin coals are vitrinite rich, composed of discrete coal and shale interbeds 2-10 ft thick at drilling depths of up to 2,500 ft, present in a stratigraphic interval at least 1,000 ft thick, that collectively form net thicknesses of 15 to 70 ft and dramatically thicken, thin, pinch out, or abut against sills or dikes in distances of less then 5,000 ft. The data basemore » for these conclusions is comprised of petrographic observations for approximately 200 coal crushed particle pellets, 54 measured sections, 294 paleocurrent measurements at 45 stations, geophysical logs for 50 wells, and observations of field depositional relationships at over 200 outcrops.« less

Development of the Endicott Field (Kekiktuk Formation), North Slope, Alaska

Endicott field, located 2 mi offshore in the Arctic Ocean, produces approximately 100 MBPD from Mississippian Kekiktuk Formation fluvial sands. The field is the first Arctic offshore development. Currently, 65 wells have been drilled from two man-made gravel drilling and production islands. Production started in 1987, and 77 MMBO have been produced to date. Original volumes in place are estimated at 1 billion STB, 365 bcf of gas-cap gas, and 750 bcf of solution gas. The Kekiktuk Formation, deposited by a southerly flowing fluvial system in response to the Ellesmerian Orogeny, rests unconformably on Franklinian (Devonian) basement. The field is a combined structural/stratigraphic trap and is delineated by three major normal faults. Within the field, the Endicott Group sediments are truncated by the regional Lower Cretaceous unconformity (LCU). A reservoir seal is provided by the overlying conformable Itkilyariak Formation carbonates and Cretaceous shales above the LCU. Due to complex reservoir geometry, a subzone approach to reservoir development, surveillance, and management has been implemented. Structural dip combined with continuous interbedded shales and sealing intrareservoir faults divide the reservoir into six separate hydrologic subzones. Additionally, structural dip creates narrow oil corridors in each reservoir subzone. All development wells are within approximately 1,500more » ft of the updip gas cap and downdip aquifer. The high priority placed on incorporating reservoir description with surveillance data has resulted in optimal field management and minimized development risk.« less

New Interpretations of Pennsylvanian and Permian Stratigraphy, San Juan Basin and Southeast Paradox Basin: ABSTRACT

The Honaker Trail, Paradox, and Pinkerton Trail Formations of the Hermosa Group are recognized throughout most of the San Juan basin. The Paradox Formation is extended southeastward beyond the limits of its evaporite facies into the basin, where it consists of thick shelf-carbonate rocks and thin black shale, sandstone, and siltstone interbeds. Where the Hermosa Group thins onto the marginal uplifts, the Paradox loses the thick carbonate rocks and becomes indistinguishable from the rest of the Hermosa. The Hermosa is correlated in the subsurface with the Madera and Sandia Formations to the southeast. The transitional Rico Formation, between the marine Hermosa Group and the continental Cutler Formation, is identified throughout the subsurface of the San Juan basin and is correlated with similar deposits out-cropping along the northern and eastern margins. The Cutler Formation includes the Organ Rock, Cedar Mesa, and Halgaito members throughout most of the basin. In the vicinity of the Hogback monocline, the Cedar Mesa Sandstone Member undergoes a gradational eastward facies change from cyclic evaporite and sandstone to thick-bedded sandstone. The subsurface Cedar Mesa is correlated in part with similar rocks in the outcropping Abo and Supai Formations.

Modelling of thermal convection in sedimentary basins and its relevance to diagenetic reactions

Mathematical calculations of thermal convection have been carried out using a porous three-player model to simulate pore-water flow in a sedimentary basin with layers of different permeabilities. The calculated flow lines demonstrate that even very thin layers (< 1 m) with low permeability, like shales in a more permeable sandstone sequence, will split potentially larger convection cells into smaller units of clean sandstone beds which may then be too small to exceed the critical Rayleigh number. In a situation where we have horizontal isotherms, a sandstone bed with 1 Darcy permeability must have a thickness of more than 330 m without shale interbeds for convection to occur. This situation is rarely met in sedimentary basins. In the case of sloping isotherms, non-Rayleigh convection will always occur but the flow rate is proportional to the effective thickness of the bed and the slope of the isotherms. Calculations suggest that the flow rates are insignificant for diagnetic reactions except in situations where we have steeply sloping isotherms eg, around salt domes and igneous or hydrothermal intrusions. Analyses of formation waters from sedimentary basins like the North Sea often show evidence of a crude stratification of the pore water with respect to salinity and isotopic composition. This is evidence suggesting that large scale convection, or other types of mixing, does not take place and positive salinity depth gradient may help to physically stabilize the formation water. The fact that many of the shallow reservoir rocks from the North Sea have formation water with low salinity and negative δ 18O values suggest that this is modified meteoric water and that migration of petroleum from deeper parts of the basin occurred as a separate phase along restricted migration pathways and was not associated with a high flux of pore water. In the absence of thermal convection the total pore water flux through sandstones will be rather small except locally where we may have focused compactional flow through small cross-sections. Diagenetic reactions will, therefore, normally be relatively isochemical during deeper burial.

Flysch trace fossils from the Cretaceous Kodiak Formation of Alaska

The Cretaceous Kodiak Formation at the northeastern end of Kodiak Island, Alaska, is interpreted to have been deposited in a distal deep-sea fan complex. Facies analysis of the sequence suggests deposition in channels (both major and distributary), lobe fringe (proximal and distal), and interchannel and interchannel/channel mouth environments. The strata contain a diverse and relatively abundant trace fossil assemblage consisting, at the ichnogeneric level, of twenty-six forms: Acanthorhaphe, Chondrites, Circulichnis, Cochlichnus, Cosmorhaphe, Desmograpton, Glockerichnus, Gordia, Helminthoida, Helminthopsis, Muensteria, Neonereites, Nereites, Paleodictyon, Palaeophycus, ?Phycosiphon, Planolites, Protopaleodictyon, Scolicia, Spirophycus, Spirorhaphe, Taenidium, Taphrhelminthopsis, Terebellina, Thalassinoides, and Yakutatia.Thirty-eight ichnospecies are described, three of which (Acanthorhaphe ichnosp., Cosmorhaphe helicoidea, and Desmograpton fuchsi) are recorded for the first time in North America. Final channel-fill (levee) and interchannel environments contain the most abundant and diverse ichnoassemblages, a reflection of favorable environmental parameters for inhabitation by benthic organisms, but more importantly the presence of diverse lithotypes conducive to the final preservation of their activity. Channel-fill sequences are devoid of trace fossils, a reflection of substrate mobility and lack of shale interbeds precluding toponomic preservation. Lobe fringe sequences contain only isolated trace fossils, a reflection of their poor exposure and largely monolithologic character.The ichnoassemblage is compared to 20 previously described and taxonomically well-documented deep-water flysch trace fossil studies from the Phanerozoic. Comparison at the ichnogeneric level is favorable with only Acanthorhaphe, Terebellina, and Yakutatia being relatively unique to the Kodiak sequence.