River Sandstone Research Articles

Implications of Outcrop Geology for Reservoirs in the Neogene Productive Series: Apsheron Peninsula, Azerbaijan

The Neogene sediments of eastern Azerbaijan and western Turkmenistan contain significant hydrocarbon reserves in deltaic reservoirs. Sedimentary logging of these reservoirs at outcrop in Azerbaijan has outlined four facies associations that encompass a range of paleoenvironments, from alluvial braided river sandstones and conglomerates to delta-front siltstones and mudstones. The facies associations suggest a river-dominated braid delta and are arranged into an architecture controlled by base-level changes: parasequences, parasequence sets, and sequence boundaries are key elements. The facies associations and stratigraphic architecture, together with outcrop observations of cementation and faulting, are summarized in terms of four idealized reservoir models: fluvial, delta plain, proximal delta front, and distal delta front. Each reservoir model has distinctive grain-size and shale distributions. Comparison of nearby oil fields with the studied outcrops suggests that the outcrops form good analogs from which to model reservoir architecture, barriers and baffles to fluid flow, reservoir heterogeneity, and reservoir quality variations.

Marine ostracods in the Lower Carboniferous fluviatile Fell Sandstone Group: evidence for base level change and marine flooding of the central graben, Northumberland Basin

SUMMARY Seven species of ostracod have been identified from the coarser, more silty laminae within a thin shaly siltstone interval within the Lower Carboniferous Fell Sandstone Group in the eastern part of the Northumberland Basin, all of which indicate a quiet, nearshore, shallow water marine environment. This is the first documented occurrence of marine ostracods from the Fell Sandstone which has always been regarded as entirely of non-marine origin, with any marine influences confined to the westernmost part of the basin. The shaly siltstone was proved in the Alnwick Borehole drilled through the upper section of the Fell Sandstone in an otherwise sandstone-dominated succession. The presence of marine influenced conditions more than 60 km inland from the coeval shoreline reflects contemporaneous intrabasinal tectonics and the location of the Alnwick Borehole at the northeastern end of the central graben within the Northumberland Basin. Marginal fault activity, contemporaneous with Fell Sandstone deposition, caused gentle tilting of the graben floor, thus allowing marine conditions to penetrate farther inland than previously known. The ostracod-bearing marine shaly siltstone, which lies above and below braided river sandstones, provides a key sequence stratigraphic surface across this part of the basin. The base of the unit is a transgressive surface with the shale itself equivalent to a maximum flooding to truncated shale prone early high stand. The immediately overlying erosion surface records low stand incision, facies tract dislocation and the development of a possible sequence boundary, with the overlying stacked, cross-bedded, coarse pebbly braided channel sandbodies referred to the lower and middle parts of a transgressive systems tract that spans the boundary between the Fell Sandstone and succeeding, more marine influenced Scremerston Coal Group. The lower, finer-grained, fluvial succession beneath the transgressive surface is tentatively assigned to part of an incompletely exposed transgressive systems tract.

Radioisotope dynamics — the origin and fate of nuclides in groundwater

The use of radionuclides as clocks for groundwater dating and as probes to investigate the geometry and spatial extent of the contact area between rocks and water is reviewed. Subsurface production rates for 222Rn 37Ar, 85Kr, 39Ar, 36CI, 3He, 4He and 40Ar in various rock types are listed. Measured Rn fluxes from the surface of sandstone grains and from pieces of granite point to scale-dependent diffusion coefficients. The temporal evolution of subsurface-produced 222Rn-, 37Ar-, 85Kr- and 39Ar-activities in groundwaters yields radionuclide escape factors between 0.1 % and 9% for the Stripa granite (Sweden) and between 1% and 4% for the Milk River sandstone (Canada). The combination of 3H, 85Kr, 39Ar, 14C, 36CI, 4He in the UK Triassic sandstone aquifer allows groundwater dating up to 40 000 a. Very old groundwaters can be studied using Cl, 36Cl and 4He evolution as demonstrated in the Milk River aquifer in Canada.

Application of Oil-Based-Mud Pressure Coring to the Determination of In-Situ Water Saturations

Summary This paper describes a field project which applied pressure coring using oil-based mud (OBM) to determine whether a surfacing correction was needed to use OBM-core water saturations (SW) as accurate values for in-situ Sw. Six OBM pressure cores were cut in two Prudhoe Bay field wells, one in the low Sw area and the other in the higher SW area. This paper discusses the equipment and procedural changes, both in the field and in the laboratory, required for this project. Pressure cores were taken from two geologically different intervals: the Sag River sandstone and the Ivishak (Sadlerochit) sandstone. The results of this project are that for the Ivishak sandstone, no water was expelled during the thawing (taken to be equivalent to core depressuring) for the full range of Sw (1 to 45%); but for the Sag River sandstone, some water was expelled for samples with Sw values of 25 to 40%. The difference in water-expulsion results is attributed to differences in pore structure. Finally, the application of these results to the calculation of in-situ Sw is discussed.

Timing of Compaction and Quartz Cementation from Integrated Petrographic and Burial-History Analyses, Lower Cretaceous Fall River Formation, Wyoming and South Dakota

ABSTRACT Integrated petrographic and burial-history studies of Fall River sandstones from outcrop and the subsurface provide insight into the timing of compaction and quartz cementation, the two main porosity-reducing processes in quartzose sandstones. Petrographic study of 95 thin sections of Fall River fluvial valley-fill sandstones from outcrop, Donkey Creek field at 2 km burial depth, and Buck Draw field at 3.8 km indicates that reservoir quality differs significantly in these three areas. Fall River sandstones at the surface contain an average of 31% intergranular volume (IGV) and 2% quartz cement. In both Donkey Creek and Buck Draw fields, the sandstones average 22% IGV, but quartz-cement volume averages 8% in the shallower field and 12% in the deeper. Geometric mean permeability at the urface is 4700 md, compared with 42 md at 2 km and 2 md at 3.8 km. Burial history of the Fall River sandstone differs greatly in the three areas. The outcropping sandstones were buried to 2 km and had reached 80°C by the end of the Cretaceous. They were then uplifted and have remained at near-surface temperatures since the Paleocene; the calculated time-temperature index (TTI) of these sandstones is 1. Fall River sandstones at Donkey Creek were also buried to 2 km and had reached 80°C by the end of the Cretaceous but remained at that depth during the Tertiary; TTI is 14. In Buck Draw field, Fall River sandstones were buried to 2.5 km during the Cretaceous and then continued to subside during the Tertiary, reaching depths of 4 km and temperatures of 140°C; TTI is 512. Much of the compaction of Fall River sandstones had occurred by burial depths of 2 km, but additional time at that depth was required for completion of mechanical compaction and for minor chemical compaction. Although greater depth and higher temperatures did not increase chemical compaction, increasing thermal maturity resulted in more quartz cementation. Internal sources of silica from stylolitization and intergranular pressure solution account for only 10-25% of the volume of quartz cement in Fall River sandstones. Imported silica may have come from clay-mineral transformations in Cretaceous shales. Loss of porosity and permeability was greatest between TTI of 1 and 14 but continued, at a reduced rate, as TTI increased.

Neogene Stratigraphy and Sedimentology in Eastern Azerbaijan: Outcrop Observations and Subsurface Implications: ABSTRACT

The largely Pliocene Productive Series of eastern Azerbaijan contains about 26 billion barrels oil equivalent. It is well exposed in outcrops on the Apsheron Peninsula which a joint team from the GIA and the BP and Statoil Alliance have described. Detailed biostratigraphic and petrographic studies have subsequently been carried out. Productive Series deposition was initiated by a dramatic relative sea-level fall which left the South Caspian an isolated basin fed by deeply incised precursors to the modern Volga, Amu Darya, and Kura rivers. Five facies associations have been recognised within the Productive Series at outcrop, encompassing a range of palaeoenvironments from alluvial braided river sandstones and conglomerates to delta-front siltstones and mudstones. The facies associations suggest a river-dominated, braid delta. Four idealised reservoir models can be recognised: fluvial, delta-plain, proximal delta-front and distal delta-front. Each has distinct grain-size and shale distributions. Studies of nearby oilfields suggest that these models form useful subsurface analogues. Flow simulation models suggest that each reservoir type has dramatically different performance. Productive Series sediments are typically loosely cemented and smectite rich, which may result in clay swelling and sand control problems. Localised reduction in reservoir quality is caused by fault-associated calcite cements. Proximal facies of themore » upper Productive Series contain porosity occluding gypsum cements. Palynology and nannopalaeontology have been applied to the Neogene sediments of Azerbaijan for almost the first time, and have given encouraging results, at least in terms of a broad biozonation. Micropaleontological analyses have also provided useful palaeoenvironmental data.« less

Depositional and Diagenetic Controls on the Distribution of Porosity in the Lower Cretaceous Fall River Sandstone: ABSTRACT

Depositional and Diagenetic Controls on the Distribution of Porosity in the Lower Cretaceous Fall River Sandstone: ABSTRACT

The Sag River Sandstone (Triassic), North Slope, Alaska: An Example of Shelf Sandstone Deposition under the Influence of Upwelling: ABSTRACT

The Sag River Sandstone (Triassic), North Slope, Alaska: An Example of Shelf Sandstone Deposition under the Influence of Upwelling: ABSTRACT

Sedimentology of the Rough Rock: a Carboniferous braided river sheet sandstone in northern England

Abstract The Rough Rock is a coarse-grained fluvial sheet sandstone which outcrops across northern and central England. It is interpreted as a braided river sandstone on the basis of sedimentary structures, internal erosion and growth surfaces. This study reveals that the Rough Rock represents more than a single phase of deposition and that internal erosion surfaces may indicate anabranch avulsions or more significant river avulsions. Detailed outcrop profiles are presented to try and distinguish the styles of deposition and the nature of internal bounding surfaces and channel geometry. The Rough Rock Formation contains a very large number of first and third order bedding contacts, which represent the migration of dunes and channels respectively, there are relatively few macroforms and bars preserved. The lack of second order surfaces within many outcrops could indicate that bars in the Rough Rock rivers were very large and therefore cannot be resolved at the outcrop scale. Similarly, the rivers which deposited the Rough Rock may have had such high width: depth ratios that the dip of second order bounding surfaces are too low to be identified. The large number of third order bounding surfaces indicates channel stacking which may have occurred in two ways: (1) relocation of rivers in the same area over time, or, (2) anabranch avulsion within a braided river within a hierarchy of channels. The problems associated with differentiating between these two mechanisms are discussed.

Oil-Base Coring For Connate Water Saturation Utikuma Keg River Sandstone

Abstract A successful oil-base coring operation has resulted in identification of an average Connate Water Saturation (Swc) at Utikuma Lake oilfield supporting low Swc from recently completed Single Well Tracer (SWT) tests. As oil-base coring was intended to validate SWT results, careful design of the coring program was required. Alteration of Swc by the coring procedure itself WAS eliminated by preserving wettability through appropriate coring fluid selection and surface handling of the core. Special attention to the drilling program design eliminated contamination by uphole formation and drilling fluids. This paper presents details of the oil-base coring program, including drilling program design, equipment and coring fluid selection core handling and core analysis results. Introduction Utikuma Lake Field is located north of the town of Slave Lake, Alberta, between the Nipisi and Red Earth fields (Fig. 1). Field characteristics are summarized in Table 1. Utikuma produces 1500 m3/d of light, sweet oil under primary water drive from 180 wells completed in the Keg River sandstone. Utikuma Keg River oil is paraffinic with low asphaltene content. Oil properties are listed in Table 2. Formation water is highly saline, with 275 000 mg/l total dissolved solids, as indicated in the water analysis data presented in Table 3. Geology Utikwna Keg River sands were derived from the Peace River Arch west of the field, being deposited in Utikuma as numerous separate pools in topographic lows of irregular highs in the eroded Precambrian granite. The porous Keg River sands were encased in impermeable shales that formed upper and lower seals. Keg River sands consist largely of interbedded sand and shale, each being several centimetres to several decimetres thick. The sands are feldspar-rich quartzose with calcite and anhydrite cementation, and fine to very coarse grains. Clays are almost entirely illite, a non-swelling clay. Average porosities and permeabilities for the Keg River sands are 20% and 1000 mD. Average net pay is 3 m. Need for Oil-base Coring Oil-base coring was undertaken at Utikuma to confirm low connate water saturations (Swcs) identified by Single Well Tracer (SWT) tests conducted during 1986–1988. Swcs of 10% to 15% from SWT tests were significantly lower than the 29% Swc recognized for most Utikuma Keg River pools. Oil-base coring was justified by the significant impact on reserves estimates and field development plans that would be caused by adopting the lower Swcs. Oil-base coring has been undertaken at two development well locations in Utikuma. The first attempt at well 5-30-81-9 WSM in December 1987, did not provide representative Swc data as the Keg River sand was determined to be entirely within a transition zone above an unexpected oil/water contact. Experience gained from well 5–30 was used in the subsequent successful oil-base coring project at well 7-33-81-9 WSM in July 1988. The 7–33 oil-base coring project is the subject of this paper. Oil-base Coring Project Design The Utikuma oil-base coring project began with selection of a project team comprising reservoir, production and drilling engineers, laboratory representatives and downhole equipment and drilling fluid suppliers.

Single Well Tracer Testing For Connate Water Saturation At Utikuma Lake: Field Application

Abstract Three Single Well Tracer (SWT) tests for Connate Water Saturation (Swc) have been successfully conducted in the Keg River Sand at the major Utikuma Lake oilfield, leading to significant reductions in estimated Swc and increases in reserves. Resistivity logs and conventional core analysis yield erroneously high Swc in the Utikuma Keg River Sand due to thinly interbedded sands and shales, and severe wettability alteration in restored core. SWT testing allows in situ measurement of Swc in productive sand intervals only, investigating saturations within a 5 m radius of investigation. The Utikuma tests represent the latest 3 of 10 tests conducted worldwide. They are the first SWT tests for Swc to use a novel downhole equipment configuration to avoid plugging by waxy crude. They also introduced the use of ethyl formate as the primary tracer for accurate measurement of Swc in 5% to 15% range. SWT results are supported by NGT/GST logs run during the programs, and the first public verification by oil base Core. Introduction Utikuma Lake Field is located north of the town of Slave Lake, Alberta, between the Nipisi and Red Earth fields (Fig. 1). Field characteristics are summarized in Table 1. Utikuma produces 1500 m3/d of light, sweet oil under primary water drive from 180 wells completed in the Keg River Sandstone. The Keg River sands are deposited in Utikuma as numerous separate pools, laid over geological unconformities in the Precambrian granite. Average porosities and permeabilities for the Keg River sands are 20% and 1000 mD. Average net pay is 3 m. Utikuma Keg River oil is paraffinic with low asphaltene content. Oil properties are listed in Table 2. Formation water is highly saline, with 275 000 mg/l total dissolved solids, as indicated in the water analysis data presented in Table 3. Connate Water Saturation Determination at Utikuma Connate Water Saturations (Swc's) of 29% based on early log and special core data have been recognized for Utikuma Keg River pools since discovery. Recognized inaccuracies in open hole log data and concerns over reliability of early core flood data led to further investigation of Swc. Accurate Swc data was required to calculate reserves and determine a development strategy for the field. Increased reserves and production gains by increasing reserves-based Maximum Rate Limitation (MRL) provided economic incentive to aggressively pursue the program of Swc determination through Single Well Tracer (SWT) testing described in this paper. Swc from Resistivity Logs Swc determination from conventional resistivity logs is hampered in the Utikuma Keg River Sandstone by thin interbedding of sands and shales, and bed thicknesses less than the 2.4 m vertical resolution of resistivity tools. Radioactive feldspars affect Gamma Ray response, preventing shaly sand analysis. Swc's of 30% to 60% are typically obtained from conventional and shaly sand log analysis in the Utikuma Keg River Sandstone. Swc from Core Utikuma Keg River sand is a feldspar-rich quartzose as indicated in the X-ray Diffraction analysis presented in Table 4.

Sequence Stratigraphy of the Fall River Sandstone, Northeastern Powder River Basin, Wyoming and Montana: ABSTRACT

Sequence Stratigraphy of the Fall River Sandstone, Northeastern Powder River Basin, Wyoming and Montana: ABSTRACT

Petrographic Differentiation of Fluvial and Estuarine Sands, Fall River Formation, Powder River Basin, Wyoming: ABSTRACT

The authors examined 170 core and cuttings samples of the Fall River Sandstone from 28 wells in three areas of the Powder River basin: Buck Draw, Silver Spoon, and East Flank. Information from thin sections on sorting, textures, grain size, detrital mineralogies, clay types, and early diagenesis were used to differentiate depositional facies and evaluate controls on porosity. Most of the porosity in the Fall River Sandstone is secondary, and its development is a function of depositional facies and geographic location.

Depositional Environments, Sequence Stratigraphy, and Trapping Mechanisms of Fall River Formation in Donkey Creek and Coyote Creek Oil Fields, Powder River Basin, Wyoming: ABSTRACT

Donkey Creek and Coyote Creek fields contain combined reserves of approximately 35 million bbl of oil and are within a trend of fields on the eastern flank of the Powder River basin that totals over 100 million bbl of reserves. The principal producing formation is the Lower Cretaceous Fall River Sandstone. A study of 45 cores and 248 logs from the three pools in the Donkey Creek and Coyote fields has shown that the Fall River is composed of three progradational deltaic units deposited during a period of rising relative sea level. These are locally eroded and are filled by a fluvial point-bar complex deposited following a lowering of relative sea level. Four important depositional facies have been recognized: the delta-front and distributary-channel sandstone of the highstand deltaic sequence and the point-bar sandstone and channel-abandonment of the lowstand fluvial sequence. Stratigraphic traps in Coyote Creek and south Donkey Creek pools are the result of permeable (250 md) point-bar sandstone (250 bbl oil/day ip) bounded updip by impermeable (0.1 md) channel abandonment mudstone. Most of the oil in the central Donkey Creek pool is produced from permeable (76 md) distributary-channel sandstone (150 bbl oil/day ip), which is restricted to the westernmore » flank of a structural nose. Lesser production, on the crest and upper western flank of the structure, is obtained from the less permeable (2.8 md) delta-front sandstone (50 bbl oil/day ip). Production is possibly limited to the crest and western flank by hydrodynamic processes.« less

Oxygen Isotope Studies of Diagenesis and Pore-Water Evolution in the Western Canada Sedimentary Basin: Evidence From the Upper Cretaceous Basal Belly River Sandstone, Alberta

ABSTRACT The oxygen isotope compositions and paragenetic sequence of diagenetic minerals from the Upper Cretaceous basal Belly River sandstone, southwestern Alberta, have been used to identify changes in pore-water composition during diagenesis. These changes can be related to major geologic events within the western Canada sedimentary basin. Low-18O meteoric water has played an important role in the diagenesis of these Upper Cretaceous rocks on a basinwide scale during deposition and early diagenesis and following uplift of the basin in early Eocene time. Diagenetic phases include early chlorite (18O = +6.1 to +8.3) and calcite (18OSMOW = +21.0; 13CPDB = -1.6), followed by quartz overgrowths (18O = + 13. 1 to + 18.0), calcite (18OSMOW = + 10.4 to + 14.4; 13CPDB = - 10.6 to - 1.2), and then kaolinite (18O = +7.8 to + 11.9), illite/smectite (18O = +10.9 to +13.1) and smectite (18O = +12.6 to +13.7). The interpretation of these results is that shallow diagenesis, early in the burial history (chlorite, calcite) occurred in the presence of brackish or fresh water. With burial, the 18O value of the pore water increased due to water/rock interaction. Burial diagenesis was terminated in the early Eocene by uplift (Laramide Orogeny). Recharge of the basin by low-18O meteoric water occurred at this time. The meteoric water then became involved in the formation of diagenetic quartz and calcite at or near maximum burial temperatures, and in the crystallization at lower temperatures of kaolinite, illite/smectite, and smectite as post-Eocene cooling and erosion progressed.

Lateral and vertical facies sequences in the Upper Cretaceous Chungo Member, Wapiabi Formation, southern Alberta

The Chungo Member of the Wapiabi Formation (Campanian) shales out northward and (or) eastward from Lundbreck to the Bow Valley, southern Alberta. At Lundbreck, basinal mudstones are overlain by turbidite and hummocky cross-stratified sandstones in an overall coarsening-upward sequence. The marine part of the sequence is capped by dominantly swaley cross-stratified sandstones, interpreted as storm-dominated shoreface deposits. Nonmarine fluvial deposits overlie the shoreface. Essentially the same sequence persists northward to the area of the Highwood River, but in the Highwood River – Sheep River – Longview area, the swaley cross-stratified shoreface sandstones are progressively cut out by a north- and (or) east-ward-dipping erosion surface with up to 40 m of relief. The erosion surface is overlain by a lag of chert pebbles and then by a marine coarsening-upward sequence of offshore to shoreface deposits (the lower tongue of the Nomad Member). Nonmarine Chungo deposits then prograded north- and (or) eastward to overlie this marine tongue in the Highwood area. They are in turn abruptly overlain by a gravel-coated transgressive surface marking the upper marine tongue of the Nomad Member. Finally, marine shales of the Nomad Member are abruptly overlain by dominantly fluvial rocks of the Belly River Formation.We suggest that the main Chungo shoreface, stratigraphically equivalent to the Milk River sandstone of southeastern Alberta, prograded north- and (or) eastward to at least the Highwood River area. A rapid relative lowering of sea level followed by stillstand resulted in the formation of the erosion surface and the supply of gravel to the basin. The overlying coarsening-upward sequence indicates the resumption of shoreline progradation, but this appears to have been relatively short-lived; it was terminated by the transgression of the upper Nomad tongue.

Initiation of petroleum exploration in Jameson Land, East Greenland

Active petroleum exploration in East Greenland is of fairly recent date and was preceded by a much longer history of scientific work and mineral exploration. The discovery in 1948 of lead-zinc mineralisation at Mestersvig resulted in the formation of Nordisk Mineselskab AIS in 1952. In the beginning of the seventies Nordisk Mineselskab initiated cooperation with the American oil company Atlantic Richfield (ARCO) in order to undertake petroleum exploration in Jameson Land. The Jameson Land basin contains a very thick Upper Palaeozoic - Mesozoic sedimentary sequence. Important potential source rocks are Lower Permian lacustrine mudstone, Upper Permian black marine mudstone, Middle Triassic dark marine limestone, uppermost Triassic black marginal marine mudstone, Lower Jurassic black mudstone and Upper Jurassic deep shelf black mudstone. Tbe Upper Permian mudstone, which is the most promising source rock, is immature to weakly mature along the western basin margin and is expected to be in the oil or gas-generating zone when deeply buried in the central part of the basin. Potential reservoir rocks include Upper Permian bank and mound limestones, uppermost Permian fan delta sandstones, Lower Triassic aeolian and braided river sandstones, and Lower, Middle and Upper Jurassic sandstones. The most important trap types are expected to be stratigraphic, such as Upper Permian limestone bodies, or combination stratigraphic-structural such as uppermost Permian or Lower Triassic sandstones in Early Triassic tilted fault blocks. In the offshore areas additional play types are probably to be found in tilted Jurassic fault blocks containing thick Lower, Middle and Upper Jurassic sandstones and lowermost Cretaceous sandstones and conglomerates. The recognition of the potential of the Upper Permian in petroleum exploration in East Greenland has important implications for petroleum exploration on the Norwegian shelf.

Abnormal Pressure, Natural Fractures, and Prolific Hydrocarbon Production in Fall River Sandstone, Deep Powder River Basin, Wyoming: ABSTRACT

Prolific hydrocarbon production from the Lower Cretaceous Fall River (Dakota) sandstone occurs in several fields in the deep overpressured portion of the Powder River basin. Most of the production to date comes from Buck Draw field at depths greater than 12,000 ft (3657 m). Individual wells in the field are capable of producing more than 2000 BOPD and have cumulatively produced in excess of 1 million bbl. Production comes mainly from stratigraphic traps. The Fall River Formation consists of fluvial, deltaic, and marine sandstones. Most of the productive sandstones in the Buck Draw area are interpreted to be fluvial (point bar) in origin. This interpretation is based upon log-curve shape, mapped subsurface trends, and the vertical sequence of sedimentary structures, lithologic textures, and mineralogic composition that was observed in cores. The fluvial sandstones were deposited by a meandering river that incised through previously deposited delta-front and marine sandstones. The delta-front and marine sandstones are in transitional contact with the underlying Fuson Shale, whereas the fluvial sandstones are in erosional contact with the Fuson. The updip traps to the fluvial sandstones are clay plugs in abandoned meander loops and the less-permeable delta-front and marine sandstones.

Oxygen Isotope Studies of Diagenesis in the Basal Belly River Sandstone, Pembina I-Pool, Alberta

ABSTRACT The oxygen-isotope compositions of authigenic clay and carbonate minerals from the Pembina I-Pool (basal Belly River sandstone) provide useful constraints on the diagenesis of this unit. Early diagenetic chlorite has 18O values (+ 4.2 to + 6.3, SMOW) compatible with low-temperature crystallization from brackish water. The 18O values of diagenetic calcite (+ 11.8 to + 14.7, SMOW) and kaolinite (+ 10.4 to + 12.8, SMOW) are best explained by crystallization at temperatures of at least 45° to 70°C from pore fluids similar in 18O to the existing formation water (-9, SMOW). The implicatio of such temperatures is that the sandstone was much more deeply buried in the past than it is at present. At least 1,300-1,400 m of overlying material may have been eroded since maximum burial.

Jurassic-Neocomian Biostratigraphy, North Slope, Alaska: ABSTRACT

The foraminiferal and palynological biostratigraphy of subsurface Jurassic and Neocomian (Early Cretaceous) age strata from the North Slope were investigated to better define biostratigraphic zone boundaries and to help clarify the correlation of the stratigraphic units in the National Petroleum Reserve in Alaska (NPRA). Through use of micropaleontologic data, eight principal biostratigraphic units have been identified. The Neocomian and Jurassic strata have each been subdivided into four main units. The gamma-ray zone (GRZ), or hot zone, sediments include both northern and southern source starved-basin deposits. These deposits range from Barremian to possibly middle Albian age in NPRA. East of NPRA, these Early Cretaceous starved-basin deposits coalesce with Late Cretaceous starved-basin deposits and form a GRZ at least as young as Senonian. Early Cretaceous starved-basin deposits of northern source are recognized by the presence of dispersed, rounded, frosted quartz sand grains. These sand grains, floating in a mudstone matrix, are essentially absent from the starved-basin deposits of southern source. The Lower Cretaceous unconformity occurs within the Hauterivian to Barremian biostratigraphic interval in the Tunalik, Peard, and Inigok wells on the NPRA; it cannot be recognized through microfossil evidence in those particular areas. In most other areas of the North Slope, the unconformity occurs at the base of the Hauterivian to Barremian biostratigraphic interval. This unconformity truncates progressively older strata eastward along the present coastal region and onto the Barrow arch. A mid-Jurassic sandstone unit appears to be of Early to Middle Jurassic rather than Late Jurassic (Oxfordian) age. Lower to Middle Jurassic strata are characterized by southward depositional thinning and distal (starved-basin) deposition in certain areas. The Sag River Sandstone is a time transgressive unit that ranges from Late Triassic in the north and northwestern NPRA to Early Jurassic on the Fish Creek platform to the east. End_of_Article - Last_Page 669------------