Lower Tertiary Rocks Research Articles

3D seismic model of faulting systems of a Jurassic - Cretaceous sedimentary packages in Merjan_West Kifl Oil fields - Central Iraq

Study of three dimension seismic data of Merjan survey, central Iraq has showed that the Jurassic – Cretaceous succession is affected by faulting system. Seven major normal faults were identified and mapped. Synthetic traces calculated from sonic and density logs of the well Me-1. Two exploration wells were drilled in the area Me-1 and Wkf-1 wells. The area is characterized by extension phase led to graben system forms rift sub-basin. The structural components of rift sub-basin has identified and visualized in three dimensions. Discussion about the effect of this system on the sedimentary package has been presented. Faulting framework can be divided into two groups: the first affects the Jurassic and lower Cretaceous rocks and the second affects the upper Cretaceous and lower Tertiary rocks. The first group is associated with the post rift thermal sag, passive margin progradation and gravitational collapse (lower Jurassic – mid Turonian 190–92 ma); approximately during the Sargelu – NahrUmr depositional time. The second group is limited and is associated with the rifting creating the Euphrates graben as collision wanes (Late Turonian – Maastrichtian 90–70 Ma) approximately during the Tanuma shale/Sadi – Shiranish depositional time.

Using Chaos and Ant Track Attributes to Recognize The Faulting Systems and Subtle Faults of a Jurassic - Cretaceous Sedimentary Packages in Merjan_West Kifl Oil Fields - Central Iraq

Study of three dimensional seismic data of Merjan area-central Iraq has shown that the Jurassic – Cretaceous succession is affected by faulting system. Seven major normal faults were identified and mapped. Synthetic traces have been calculated by using sonic and density log data of the well Me-1.Two exploration wells were drilled in the area Me-1 and Wkf-1 wells, the distance between them is 15.82 km. Discussion about the effect of this system on the sedimentary package has been presented. The tight faults that couldn’t be distinguished it on seismic sections were determined using seismic attributes. They have different strike and limited in their vertical and horizontal extension. They are system facilitates the movement or migration of the fluid across the stratigraphic column in the study area. Faulting framework can be divided into two groups: the first affects the Jurassic and lower Cretaceous rocks and the second effect the upper Cretaceous and lower Tertiary rocks. The first group is associated with the post rift thermal sag, passive margin progradation and gravitational collapse (lower Jurassic – upper Cretaceous (Turonian) 022 – 93 Ma); approximately Sargelue – NahrUmr depositional time. The second group is few and is associated with the rifting creating the Euphrates graben (Late Turonian – Maastrichtian 90 – 70 Ma) approximately Tanuma shale / Sadi – Shiranish) depositional time.

E&P Notes (March 2015)

E&P Notes Pushing the Limits of Deepwater Stimulation Trent Jacobs, JPT Technology Writer The lower tertiary formation found in the pre-salt layers of the Gulf of Mexico has become a proving ground for extending what is possible when completing multistage fracturing in ultradeepwater wells. The stimulation vessels that pump fracturing fluids, acids, and proppant into these wells are key to achieving the volume of production needed to make them profitable. Baker Hughes operates one of the industry’s youngest fleets of such vessels in the North Sea and offshore west Africa, Brazil, and Vietnam. But because of the strength of lower tertiary rocks and the pressure required to improve contact between the well and the formation, some of the company’s newest and most advanced vessels are reserved for the Gulf. Automation Program Looks Outside Industry for Inspiration Jack Betz, JPT Staff Writer The University of Texas at Austin (UT) will launch an initiative in April to promote cross-discipline collaboration and student research on automated technology in the oil field. Called Rig Automation and Performance Improvement in Drilling (RAPID), the initiative will enrich UT’s existing drilling research by connecting petroleum-oriented researchers with other engineers and scientists in disciplines outside the oil and gas industry, which have benefited from sophisticated automation. These fields include aerospace, mechanical engineering, and computer engineering. Dual-Gradient Drilling Debuts in Gulf of Mexico Jack Betz, JPT Staff Writer Statoil has successfully used dual-gradient drilling for the first time ever in the US Gulf of Mexico (GOM) on an upper-hole section. During the trial, Enhanced Drilling’s EC-Drill system successfully maintained a desired bottomhole pressure by controlling mud volume in the riser and the rate of its return to the surface. The technique is called controlled mud level. “We drilled for about 10,000 ft on the first [GOM] well, and the system ran for 600 hours,” said John-Morten Godhavn, a managed pressure drilling specialist at Statoil, at a recent SPE Gulf Coast Section event.

Pb, Sr isotopes in organic matter of Lower Tertiary hydrocarbon source rocks in Liaohe Fault Depression

The Pb, Sr isotopic compositions in organic matters (kerogen and soluble organic matter) of Lower Tertiary hydrocarbon source rock in Liaohe Fault Depression is first reported. There is a great variable rang of Pb isotopic compositions in kerogen,206Pb/206Pb = 17.488–18.656,207 Pb/204 Pb = 15.388–15.692,208 Pb/204 Pb = 37.656–37.484. And soluble organic matters have a very stable Pb isotopic composition, that is206Pb/204Pb = 17.424–17.601,207Pb/204Pb = 15.376–15.448,208Pb/204Pb = 37.620–37.870. Sr isotopic composition varies in the range of 0.747 59–0.716 79 in kerogen and of 0.709 67–3.747 59 in soluble organic matters that have a great range. It is shown that mantle fluid can influence organic matters and make them form hydrothermal hydrocarbon in the geoglogic environment of rift basin.

Paleomagnetic study of upper Miocene rocks from northern Chile: Implications for the origin of Late Miocene‐Recent tectonic rotations in the southern Central Andes

Paleomagnetic studies in the southern Central Andes have shown the widespread presence of clockwise vertical‐axis rotations. Rock units sampled in these studies, however, are heterogeneously distributed in stratigraphic age with most paleomagnetic data from northern Chile being from Mesozoic and lower Tertiary rocks, whereas most data in the southern Altiplano, Puna, and Cordillera Oriental are from upper Tertiary rocks. In this paper we present the results of a paleomagnetic study on upper Miocene sedimentary rocks and ignimbrites from the Precordillera of northern Chile (at 22°S). These rocks are coeval with the initiation of crustal shortening in the eastern foreland fold‐thrust belt which some tectonic models relate to oroclinal rotation of northern Chile. Primary magnetizations in rocks from widely distributed sites in two ignimbrites indicate that no relative rotations have occurred between sites, suggesting the study area has acted as a single coherent block with respect to vertical‐axis rotational deformation. Although minor inadequate sampling of paleosecular variation can affect our data set, its time‐averaged paleomagnetic direction is indistinguishable from the expected late Miocene reference direction indicating no paleomagnetically significant rotation of the study area since circa 11 Ma. This suggests that late Miocene‐Recent oroclinal rotation of the northern Chilean forearc, if present, must be either very low or nonuniform. A direct implication of this result is that much of the unquestionable tectonic rotations detected from upper Miocene rocks in the southern Central Andes is of local origin. Available structural data permit us to relate several of these rotations with shear in dextral transfer zones in the foreland thrust belt on the east side of the Andes.

Geochemical delineation of the Cretaceous/Tertiary boundary in some New Zealand rock sequences

Abstract Calcium carbonate and other constituents of Upper Cretaceous and lower Tertiary rocks were determined at three localities in New Zealand where the Cretaceous/Tertiary boundary was believed to occur. The boundaries could be delineated by chemical means alone, thus reducing the time expenditure needed for the more usual paleontological techniques. This method might also be useful in locating the boundary where paleontological evidence is missing or inconclusive. Diagnostic elements were: arsenic, calcium, chromium, cobalt, iron, magnesium, manganese, nickel, phosphorus, and sulphur. In lacustrine sediments near Rewanui, a possible location of the K/T boundary was indicated by anomalous levels of calcium, iron; magnesium, and manganese. The principles enumerated should be of general application elsewhere in the world where searches continue for K/T boundary material.

Interior Provinces in Alaska: ABSTRACT

The U.S. Geological Survey is using a multidisciplinary approach to assess the petroleum potential of the interior provinces of Alaska. Geophysical parameters, petroleum geochemistry, thermal maturation, paleontology, and petrography are some of the disciplines being used to evaluate the basins as well as to compare them geologically with other explored basins. Three types of interior provinces have been tested by exploratory drilling for their petroleum potential: three Tertiary nonmarine basins, two Jurassic and Cretaceous flysch and fold belts, and a Paleozoic thrust belt. Although the presence of hydrocarbons has not yet been demonstrated, the present data base is too limited to make a definitive assessment of hydrocarbon potential. During the 1983-84 field seasons, we acquired new gravity data and collected rock samples in and adjacent to the Yukon flats and the Nenana basins. These basins contain upper Tertiary, primarily nonmarine, sedimentary rock in extensional graben and half-graben complexes that are superimposed across preexisting terrane boundaries. The location and development of the basins result from strike-slip motion along the Tintina and Denali fault systems. Adjacent to the basins and within the fault systems are thick sections of nonmarine lower Tertiary coal-bearing rocks in deformed basin remnants. If these lower Tertiary rocks are present beneath the upper Tertiary fill, their greater depth and advanced maturation could enhance the hydrocarbon generative potential. Gravity modeling suggests th Tertiary fill is at least 3 km thick in the deeper parts of the basins and may be significantly thicker. End_of_Article - Last_Page 667------------

Depositional Environments, Diagenesis, and Hydrocarbon Potential of Nonmarine Upper Cretaceous and Lower Tertiary rocks, Eastern Uinta Basin, Utah: ABSTRACT

Core studies of nonmarine rocks from the Natural Buttes field, Utah, indicate that depositional environment and diagenetic alteration control the geometry and quality of low-permeability gas reservoirs in the eastern part of the Uinta basin. The Tuscher Formation (Upper Cretaceous) is composed of fine to medium-grained, moderately to well-sorted sandstones and less abundant carbonaceous and coaly shale that formed on the lower part of an alluvial braidplain. The Wasatch Formation (Paleocene and Eocene) unconformably overlies Cretaceous rocks and consists of fine-grained lenticular cross-bedded sandstones, argillaceous siltstones, and variegated mudstones, which were deposited in lower delta-plain settings along the margin of Lake Uinta. Cretaceous and Tertiary sandstones ave been modified by minor quartz overgrowths, by the precipitation and subsequent dissolution of ferroan and nonferroan calcite, by poikilotopic anhydrite, and by the formation of authigenic illite, mixed-layer illite-smectite, kaolinite, chlorite, and corrensite. Most authigenic carbonate and anhydrite formed during early burial, before significant compaction. During later stages of diagenesis, precipitation of authigenic clay in secondary pores created by carbonate dissolution reduced porosity and permeability. Large amounts of natural gas generated in situ are stratigraphically trapped in these lenticular, diagenetically modified sandstones. Source rocks in the Tuscher Formation have reached the advanced stages of thermogenic gas generation (0.7% Ro) but are only moderatel mature with respect to liquid hydrocarbon generation. Interbeds of lacustrine Green River shale are in the early stages of gas generation (0.5% Ro) and are source rocks for gas produced from the Wasatch Formation. End_of_Article - Last_Page 860------------

Economic Potential for Upper Cretaceous and Lower Tertiary Rocks Near Helper, Carbon County, Utah: ABSTRACT

Exceptionally good exposures of Late Cretaceous and early Tertiary rocks in the western Book Cliffs area near Helper, Utah, allow detailed study of nearshore clastic sediments. The Mancos Shale is an organic-rich, silty shale, interrupted regularly by tongues of fine to medium-grained sandstone that thin to the east. The Garley Canyon and Emery Sandstones, which are the two main sandstone members in this area, exhibit a prograding clastic shoreline sequence from open-marine and lower shoreface to upper shoreface environments. Thinner sandstones also crop out in the area that indicate minor pulses of deltaic progradation and exhibit one or more of these facies. These sandstones pinch out eastward, and are offset by normal faults to the west; therefore, hydrocarbon accumula ion is likely to occur in areas to the west. The coal-bearing Blackhawk Formation is also well exposed within the study area, and is interpreted as a wave-dominated delta complex. Coal-forming swamps were situated directly on beach ridges. Several of these economically important coal seams pinch out westward (landward) within the study area. This stratigraphic interplay between terrestrial sedimentation and the coal-forming swamp environment provides details for refining coal exploration models. Upon completion of regional stratigraphic analyses, thickness variations in Upper Cretaceous and lower Tertiary formations may provide a more precise indication for the time of crustal uplift associated with the San Rafael swell. End_of_Article - Last_Page 934------------

Relationships of Source Rock, Thermal Maturity, and Overpressuring to Gas Generation and Occurrence in Low-Permeability Upper Cretaceous and Lower Tertiary Rocks, Greater Green River Basin, Wyoming, Colorado, and Utah: ABSTRACT

Most hydrocarbon production from low-permeability Upper Cretaceous and lower Tertiary reservoirs in the Greater Green River basin of Wyoming, Colorado, and Utah is gas. The most likely sources of the gas are the interbedded coal beds and other carbonaceous lithologies. A source-rock evaluation of these rocks indicates predominantly humic, type III organic matter capable of generating mainly gas. The relatively closed nature of these low-permeability rocks facilitates examination of the geologic processes involved in gas generation and occurrence. All gas accumulations are associated with overpressuring. Thermal generation of gas is the main cause of overpressuring and is directly related to organic richness, level of organic maturation, and temperature. Distances of gas migration, in most areas, do not exceed a few hundred feet. Consequently, the temporal relationships of gas generation and migration with respect to the development of structural and stratigraphic traps are not as important as in more conventional reservoirs. On the basis of the premise of minimal gas migration, the initiation, or threshold of significantly large volumes of thermogenic gas occurs at a temperat re of about 190°-200°F (88°-93°C) and a vitrinite reflectance of about 0.80 Ro. End_of_Article - Last_Page 940------------

Use of Gamma-Ray Log for Locating Cretaceous-Tertiary Unconformity, Pinedale Area, Northern Green River Basin, Wyoming: ABSTRACT

Computer-generated crossplots of the gamma-ray log versus depth in the Pinedale area, northern Green River basin, Wyoming, exhibit an abrupt shift in API gamma units over the structurally influenced depth range of 7,100-8,500 ft (2,100-2,600 m). Higher API units are recorded for all lower Tertiary rocks than for underlying Upper Cretaceous rocks. The shift, which is not obvious from viewing analog prints, coincides with the Cretaceous-Tertiary boundary as located in this area by previous workers using different methods. Evaluation of gamma-ray spectral logs from five wells, combined with petrographic and x-ray diffraction analysis of core samples from the Pinedale area, shows that this shift is caused by a higher potassium content in Tertiary rocks than in Cretaceous rocks, due to the presence of arkosic sandstones. Within the lowermost 600 ft (180 m) of the Tertiary, the potassium feldspar content of the sandstones gradually decreases with depth down to the Tertiary unconformity and then abruptly declines, essentially to zero, in the Cretaceous. Crossplots of gamma-ray intensity versus depth from other wells in the northern Green River basin show a diminution of this shift to the west and southwest of the Pinedale area. This relation suggests that the Precambrian crystalline core of the Wind River Range, which was upthrust and eroded during the Laramide orogeny, provided the source of the arkosic Tertiary sandstones. In many Rocky Mountain basins, identification of the Cretaceous-Tertiary boundary is problematical due, in part, to the similar lithology and depositional environments of the nonmarine rocks as well as to the absence of outcrops and reliable biostratigraphic data. The approach described may be applicable to other areas. End_of_Article - Last_Page 946------------

Depositional Relations of Cretaceous and Lower Tertiary Rocks, Northeastern Alaska

Analysis of depositional environments, new paleontologic data, and analogy with depositional patterns observed in areas to the west all indicate the need for revision of Cretaceous and lower Tertiary stratigraphy in northeastern Alaska. In the Sadlerochit Mountains area of the Arctic National Wildlife Refuge, the northern-derived (Ellesmerian), late Neocomian Kemik Sandstone Member and organic-rich pebble shale member of the Kongakut Formation unconformably overlie Jurassic and Triassic rocks. The unconformity, which is of mid-Neocomian age, is present throughout northernmost Alaska and passes southward into a conformable shelf sequence. After pebble shale deposition, the depositional pattern is simply one of progradational basin filling from a southern (Brookian) provena ce. This pattern is represented in vertical sequence initially by deep-marine basinal deposits succeeded by prodelta slope shales, and ultimately by deltaic deposits that prograded to the east or northeast in a predictable fashion over most of the area. The better known, thick, shallow-marine and nonmarine units in the central North Slope grade to thin prodelta or basinal turbidites; some units are entirely missing owing to nondeposition on the south-dipping flank of the basin. No evidence exists for subaerial erosion to explain these marked stratigraphic changes. In the Sadlerochit Mountains area, which may be an extension of the Barrow arch, the pebble shale (Neocomian) is overlain by about 1,600 ft (500 m) of deep-water deposits of Late Cretaceous age; Aptian and Albian rocks are either absent (by nondeposition) or are represented by a thin, condensed section. Here the Upper Cretaceous to lower Tertiary section consists of basinal shale, bentonite, and thin-bedded prodelta turbidites. Two noteworthy features in this sequence are an interval of organic-rich shale of probable Turonian to Coniacian age just above the pebble shale, and the placement of the Cretaceous-Tertiary boundary within deep-marine deposits. Unlike the western part of the North Slope, where deltaic deposition occurred in the Early Cretaceous, prograding deltaic deposition d d not reach the western part of the Wildlife Refuge until early in the Tertiary. The consistent pattern of easterly deltaic progradation is complicated at Igilatvik (Sabbath) Creek (in the north-central Wildlife Refuge) by the occurrence of thick, regressive, dominantly nonmarine deposits of early Tertiary age, which apparently prograded into the basin from the south or southeast.

Palaeomagnetism of nine dated Phanerozoic dykes in south-east Greenland

Summary Nine basic dykes were sampled near Angmagssalik, east Greenland. Specimens have been treated by alternating field demagnetization in 11 steps up to 3000 (peak) oersted (300 ml). The ‘cleaned’ direction at all sites is recognized after treatment at 150 oersted. All specimens are reversely magnetized. The mean of the site mean directions has declination = 182°.0, inclination =−66°.9, it = 45, α95= 7°.7. This direction yields a palaeomagnetic pole (reversed) at 73°.4N, 139°.5E (dp= 10°.7, dm= 12°.9) which is near, but significantly different from, that derived from lower Tertiary rocks in Greenland, namely 63°.2N, 184°.6E (A95= 4°.5). K-Ar ages of the nine dykes, based upon whole-rock and mineral separates, range from mid-Tertiary to Cambrian. It is impossible to reconcile these ages with the palaeomagnetic results. The palaeomagnetic evidence, supported by geological inference, suggests that all nine dykes are members of the east Greenland lower Tertiary dyke swarm, designated THOL1, of probable age c. 52 Ma. The difference between the poles given above can be explained by supposing that the sampling area has tipped about a horizontal axis directed along 013°/193°, the angle of rotation being 13° (± 11°) anti-clockwise, when the axis is viewed along 013°. This local effect could have been due to block faulting when the north-east Atlantic started to open, or may be attributed to upwarping of the coast due to the weight of the ice-cap inland.

Geographic distribution and dispersal of normapolles genera in North America

Normapolles pollen have been found in North America in Cretaceous and Lower Tertiary rocks from the eastern Atlantic Seaboard, the Mississippi embayment region and from the states and provinces from western North America as far north as the District of Mackenzie, Northwest Territories. Previous postulates relating to the Normapolles floral province (western Europe—eastern North America) were re-examined in the light of new finds of Normapolles genera in rocks from west of the Cretaceous epeiric seaway which separated the Normapolles province from the western North American Aquilapollenites province. A study of published occurrences of Normapolles genera and U.S. Geological Survey Denver Laboratory Normapolles records revealed that of the approximately 60 Normapolles genera recognized from western Europe, only 26 of these have been recognized from eastern North America. These data suggest that Normapolles-producing plants originated in western Europe and migrated to eastern North America prior to the opening of the north Atlantic seaway. Ten of these 26 genera also have been found in rocks from west of the Cretaceous epeiric seaway, suggesting that these genera were the only ones able to cross this barrier. At least six genera having Normapolles characteristics occur in eastern North America but have not yet been recorded from Europe. Two additional genera with Normapolles characteristics have been reported only from the Aquilapollenites province of western North America. Several discrepancies in the record need resolution, such as the latitudinal restriction of Thomsonipollis and Nudopollis to areas south 40°N latitude, the absence of records of Thomsonipollis east and north of central Georgia, and the absence of records of Kyandopollenites and Choanopollenites west of eastern Texas. These data show that the known boundaries of the Normapolles province are somewhat hazy and that firm conclusions regarding the geographic distribution and history of dispersal of Normapolles genera are premature.

Petroleum Geology of Cook Inlet Basin -- An Exploration Model

Oil exploration commenced onshore adjacent to lower Cook Inlet on the Iniskin Peninsula in 1900, shifted with considerable success to upper Cook Inlet from 1957 through 1965, then returned to lower Cook Inlet in 1977 with the COST well and Federal OCS sale. Lower Cook Inlet COST No. 1 well, drilled to a total depth of 3,775.6 m, penetrated basinwide unconformities at the tops of Upper Cretaceous, Lower Cretaceous, and Upper Jurassic strata at 797.1, 1,540.8, and 2,112.3 m, respectively. Sandstone of potential reservoir quality is present in the Cretaceous and lower Tertiary rocks. All siltstones and shales analyzed are low (0 to 0.5 wt. %) in oil-prone organic matter, and only coals are high in humic organic matter. At total depth, vitrinite readings reached a maximum ave age reflectance of 0.65. Several indications of hydrocarbons were present. Oil analyses suggest that oils from the major fields of the Cook Inlet region, most of which produce from the Tertiary Hemlock Conglomerate, have a common source. More detailed work on stable carbon isotope ratios and the distribution of gasoline-range and heavy (C12+) hydrocarbons confirms this genetic relation among the major fields. In addition, oils from Jurassic rocks under the Iniskin Peninsula and from the Hemlock Conglomerate at the southwestern tip of the Kenai lowland are members of the same or a very similar oil family. The Middle Jurassic strata of the Iniskin Peninsula are moderately rich in organic carbon (0.5 to 1.5 wt. %) and yield shows of oil and of gas in wells and in surface seeps. Extractable hydrocarbons from this strata are similar in chemi al and isotopic composition to the Cook Inlet oils. Organic matter in Cretaceous and Tertiary rocks is thermally immature in all wells analyzed. Oil reservoirs in the major producing fields are of Tertiary age and unconformably overlie Jurassic rocks; the pre-Tertiary unconformity may be significant in exploration for new oil reserves. The unconformable relation between reservoir rocks and likely Middle Jurassic source rocks also implies a delay in the generation and expulsion of oil from Jurassic until late Tertiary when localized basin subsidence and thick sedimentary fill brought older, deeper rocks to the temperature required for petroleum generation. Reservoir porosities, crude oil properties, the type of oil field traps, and the tectonic framework of the oil fields on the west flank of the basin provide evidence used to reconstruct an oil migration route. The route is inferred to commence deep in the truncated Middle Jur ssic rocks and pass through the porous West Foreland Formation in the McArthur River field area to a stratigraphic trap in the Oligocene Hemlock Conglomerate and the Oligocene part of the Tyonek Formation at the end of Miocene time. Pliocene deformation shut off this route and created localized structural traps, into which the oil moved by secondary migration to form the Middle Ground Shoal, McArthur River, and Trading Bay oil fields. Oil generation continued into the Pliocene, but this higher API gravity oil migrated along a different route to the Granite Point field.

Evaluation of organic matter, subsurface temperature and pressure with regard to gas generation in low-permeability upper cretaceous and lower tertiary sandstones in Pacific Creek area, Sublette and Sweetwater counties, Wyoming

Investigations of a sequence of Upper Cretaceous and lower Tertiary rocks in the Pacific Creek area of the northeastern Green River Basin show that studies of organic matter content, type, and maturity in conjunction with subsurface temperature and reservoir pressure, will help define prospective gas-saturated intervals and delineate areas of maximum gas-resource potential. The onset of overpressuring occurs at about 11,600 ft (3,500 m) , near the base of the Upper Cretaceous Lance Formation. Drill stem test data indicate that at about 12,800 ft (3,900 m) the pressure gradient is as high as 0.94 psi/ft (19.0 kPa/m). The development of overpressuring may involve several processes; however, interpretation of the available information indicates that active generation of large amounts of wet gas is one of the more important processes. Nearly coincident with the top of overpressuring is a reversal of the spontaneous potential (SP) curve that we believe is caused by a reduction of formation water salinity. We Interpret that the very small amounts of water produced during thermochemical decomposition of organic matter and the dehydration of clays during clay transformation may provide enough low-salinity water to effectively dilute the original formation water to a degree that the formation water resistivity greater than the mud filtrate resistivity. Microscopic and geochemical evaluation of organic matter shows that the analyzed samples contain cominantly humic-type kerogen. Total organic carbon contents of 26 samples range from 0.25 to 7.84 weight percent. Most samples exceed 0.5 percent organic carbon and the average is 1.38 percent. A vertial profile of organic maturation, determined by vitrinite reflectance and color alteration of organic material, shows that the top of overpressuring and beginning of important wet-qas generation occur at vitrinite reflectance values of 0.74 to 9.25 and color alteration values of about 2.8. The present minimum temperature at the top of overpressuring is at least 190°F (88°C) as determined from uncorrected bottom-hole temperatures recorded during logging of wells drilled in the area. The preservation of abnormally high pressures is probably due to presently active generation of gas in a thick interval of discontinuous, very low-permeability shales, siltstones, and sandstones.

Evaluation of Organic Matter and Subsurface Temperature and Pressure with Regard to Gas Generation in Low-Permeability Upper Cretaceous and Lower Tertiary Sandstones in Pacific Creek Area, Sublette County, Wyoming: ABSTRACT

Investigations of Upper Cretaceous and lower Tertiary rocks in the Pacific Creek area of the northeastern Green River basin show that studies of organic matter content, type, maturity, subsurface temperature, and reservoir pressure will help define prospective gas-saturated intervals and delineate areas of maximum gas-resource potential not included in previous U.S. Geological Survey resource estimates. The onset of overpressuring occurs at about 11,600 ft (3,500 m), near the base of the Upper Cretaceous Lance Formation. Drill-stem test data indicate that at about 12,800 ft (3,900 m) the pressure gradient is as high as 0.84 psi/ft (19.0 kPa/m). Current data indicate that the active generation of large amounts of wet gas is important to the development of this overpressuring. A reversal of the spontaneous potential curve is nearly coincident with the top of overpressuring and is probably caused by a reduction of formation-water salinity. Very small amounts of water produced during thermochemical decomposition of organic matter and the dehydration of clays may provide enough low-salinity water to dilute effectively the original formation water, so that the resistivity of the formation ater (Rw) is greater than that of the mud filtrate (Rmf). Humic-type kerogen dominates the organic matter. Total organic carbon contents range from 0.25 to 7.84 wt. %, averaging 1%. The top of overpressuring and beginning of important wet-gas generation occur at vitrinite reflectance values of 0.76 to 0.84 and color alteration values of about 2.8 on a scale of 1 to 5. The present minimum temperature at the top of overpressuring is 190°F (88°C; determined from uncorrected bottomhole temperatures). The preservation of abnormally high pressures is probably due to presently active generation of gas in a thick interval of discontinuous, very low-permeability shales, siltstones, and sandstones. The U.S. Geological Survey is studying gas-bearing intervals in the very few wildcat penetrations of deep tight sandstone intervals in Rocky Mountain basins to better define a possible major gas resource. End_of_Article - Last_Page 738------------

Geology of Gu-Dao Oil Field and Adjacent Areas: ABSTRACT

Gu-Dao oil field is located geologically in the center of Zhan-Hua basin, Bohai Bay hydrocarbon-bearing province, and geographically in the coastal zone of the lower Yellow River valley. Zhan-Hua basin is a Cenozoic block-faulted basin, with an area of 2,100 sq km. The intensive subsidence of the faulted blocks was followed by deposition of thick Tertiary continental strata (the Eocene series itself is as thick as 4,500 m), at a sedimentation rate of 0.127 mm/year. The strong block faulting in the basin led to the formation of buried Paleozoic hills, which controlled many overlapping Tertiary structures. Almost 90 of the known oil reservoirs are in these structures. Gu-Dao oil field produces from one of the late Tertiary overlapping structures which is cut along the north and south side by two faults striking nearly east-west. These faults controlled the origin and the development of the comparatively intact structure. A long period of faulting caused vertical migration of large quantities of oil and gas, and the formation of a series of multiple pays. The hydrocarbons are distinctly zoned. That is, in the Paleozoic and lower Tertiary rocks, they are high-paraffin crude oil of high wax and low sulfur content. The upper Tertiary rocks contain highly viscous aromatic-cycle alkane crude oil of low wax, high sulfur content, and dry gas occurs in the upper part of the Neogene and Quaternary. The analysis of the data shows that hydrocarbons are derived from the same source rock--the lacustrine lower Tertiary Sha-He-Jie formation--distributed in the depression surrounding the Gu-Dao structure. A secondary reservoir was formed along the faults during the multiple tectonic movement during the late Tertiary, as a result of the upward migration of hydrocarbons from lower Tertiary rocks. Hence the Gu-Dao oil field is a combination of both primary and secondary reservoirs. The main oil-bearing formation in the Gu-Dao oil field, the upper Tertiary Guan-Tao formation, comprises a set of fluvial deposits, with channel sand bodies as the principal reservoirs. The reservoir extends along the long axis of the Gu-Dao structure, and constitutes the major producing area. The oil field was discovered in 1968 and full production was begun in 1971. The development policy of water flooding at an early stage, either internally, separately, or quantitatively, was adopted according to reservoir characteristics of high viscosity, sand-out, or differences in pays. A series of measures has been taken to control sand and to reduce viscosity. Although 7 years have elapsed since the beginning of production, present oil production per well is equivalent to that of the early stage, a rising trend in productivity is obvious, and good development effects are insured. End_of_Article - Last_Page 698------------

Cretaceous and lower tertiary stratigraphy of the coastal strip between buttress point and ship creek, South Westland, New Zealand

Abstract In South Westland upper Cretaceous and lower Tertiary rocks are found in a narrow coastal strip between Buttress Point and Ship Creek. The oldest unit, the Otumotu Formation (?M-R), is derived wholly from the Greenland Group, and consists of massive breccia-conglomerate in a maroon sandy matrix (lower member) overlain by grey sandstone, greywacke conglomerate, and carbonaceous shale (upper member). The overlying Tauperikaka Coal Measures (Mh) and shallow marine Whakapohai Sandstone (Mh) are both derived from a granitic source. In the southern part of the area, Whakapohai Sandstone is overlain by Arnott Basalt (Mh-Dt}, consisting of tuff, volcanic breccia, and flows of aphyric, mildly alkaline basalt; and Buttress Conglomerate (Mh-Dt), composed of boulders of andesite and dacite with minor Greenland Group, locally interbedded with a dacite flow. The younger Tokakoriri Formation (Dt-w) consists of calcareous mudstone and sandstone, and is conformably overlain by the Abbey Formation (Dw-Lwh), which is subdivided into three informal members: pure foraminiferal limestone (lower Abbey limestone), muddy foraminiferal limestone (middle Abbey limestone), and calcareous mudstone (upper Abbey mudstone). Basaltic volcanism was continuous from Paleocene to late Eocene time near the mouth of the Paringa River, and Otitia Basalt (?Dw-Ar) consists of tuff, volcanic breccia, and flows of alkali basalt (with minor hawaiite and trachyandesite) interbedded with the Abbey Formation.

Pre-hawera geology of the Kaka District, North-West Nelson

Abstract In the Kaka District the Cretaceous Separation Point Granite is unconformably overlain by Lower Tertiary sediments belonging to the Westhaven Group which is divided into four formations. The oldest formation, Motupipi, consisting of coal measures overlain by marine sands, is conformably overlain by algal limestone and sandstone of the Huia Formation. The Huia Formation is conformably overlain by massive fine-grained sediments of the Sherry River Formation which encloses a sequence of graded beds in the north, mapped as the Burmeister Formation. The contact of Lower Tertiary sediments and granite is depositional in the south, but in the north it is faulted along the Matariki Fault Zone. Faulting was preceded by folding of the Tertiary beds into two north-north-east trending folds, the Hope Anticline to the west and the Sherry Syncline to the east. Both the granite and Lower Tertiary rocks are unconformably overlain by non-marine beds belonging to the Tadmor Group. The Tadmor Group consists of the Upper Tertiary Glenhope Formation and the conformably overlying Lower Pleistocene Moutere Gravel. In the valleys a sequence of Upper Pleistocene aggradational gravels were deposited.