Deep-water Lake Research Articles

Hydrocarbon Potential of Intracratonic Rift Basins: ABSTRACT

Significant world oil reserves have been added in recent years from rift systems. Examples of petroliferous rift basins may be found on nearly every major continent. As our understanding of the mechanisms of sedimentation and structure in rift basins grows, more rift systems will be found. With a few notable exceptions, rifts that have been explored in the past are those that formed along continental margins. These contain marine sediments, and the conditions of source rock, sediment type, depositional environment, and structural style are well-known exploration concepts. Intracratonic rift systems containing continental sediments have received little exploration effort because few have been recognized, and also because of the problems perceived to accompany continental sedimentation. A good modern analog is the East African rift system. The source rock is lacustrine shale with an organic content that ranges from 5 to 20%. The organic materials are preserved by anoxic conditions of deep lake waters. Heat flow, as in continental-margin rifts, is moderate to high. Combined with a commonly thick section and depth of burial, the sediments can be well within the oil generation window for lacustrine shales. The volume of oil generated may be very large for a basin of limited areal extent. The oil is generally waxy, has an API range from the 20s to 30s, and h s a low sulfur content. The reservoir quality is highly dependent on the type of sediments deposited, because there is little energy available for sorting or winnowing. Possibilities include first-cycle arkoses derived from crystalline basement rock and second-cycle or multicycle sands derived from earlier pre-rift depositional episodes. Eolian sands are also possible reservoirs. There may also be sharp facies variations across the rift, and aspect ratios of these facies may approach 1:1. Seals for the reservoirs are either lacustrine shales or evaporites deposited under hypersaline, closed drainage conditions. Structures are genetically similar to those found in continental margin rift valleys. Accumulation zones are found in series of tilted blocks controlled by listric, down-to-the-ba in faults; in reverse drag anticlinal features on the downthrown side of growth faults; in basement high blocks with a sedimentary End_Page 1199------------------------------ cover; and if the basin is asymmetrical, in monoclines developed on the slope. Basin size is typically 20-60 km (12-37 mi) in width and 70-300 km (43-186 mi) in length. An intracratonic rift of this type is part of the Central African rift system. This system trends east-west, from the Benue trough in Nigeria to the Lamu embayment in Kenya. It is the result of the propagation of rifting from the triple junction that separated South America from the western margin of Africa, and also from the triple junction on the eastern margin where Madagascar separated from Africa. The rift system was tectonically active during the Cretaceous and Tertiary (Paleogene), and ceased after the initial faulting and coincident igneous activity but before any new crust was formed. The rifting along this trend was superseded in the Miocene by the East African rift system, which is still active. Several companies have made significant oil discoveries in different components of the Central African rift system. Average daily production for 1982 from the basins associated with the Benue trough was 107,928 BOPD. Conoco has drilled at least eight discovery wells in the Chari basin and Ngaoundere rift components, and zones tested flowed up to 1,900 BOPD. In the Abu Gabra rift component, where Marathon is currently exploring, Chevron has drilled approximately 60 wells. Nineteen of these were discoveries and tested an average rate per well of 3,500 BOPD. The oil in the Chari basin and Abu Gabra rift is found in multiple zones in the Upper Cretaceous and Tertiary continental sands, and is a typical derivative of lacustrine sediments. The Abu Gabra rift may contain up to 10 billion bbl of oil. Research indicates that this type of rift system is present in other areas of the world. Ongoing worldwide exploration has shown that intracratonic rift basins have the potential to make a significant contribution to world oil reserves. End_of_Article - Last_Page 1200------------

Lake Ontario Sediment - Mirex Relationships

Sediment samples collected from within the eastern basin of Lake Ontario, the Oswego River and harbor indicate that mirex, a persistent, chlorinated hydrocarbon continues to accumulate in harbor and offshore lake sediments. A high statistical relationship exists between mirex and organic carbon concentrations in deep water lake sediments. Mirex-contaminated sediments are accumulating in the deeper waters (>100m) of the lake at rates ranging from about 2.2–7.0 mm/yr. Nearshore bottom sediments are subject to redistribution, providing a continuing source of mirex to lake organisms.

Hydrocarbon Occurrences in Triassic-Jurassic Lacustrine Deposits, Newark Rift Basin, Atlantic Passive Margin: ABSTRACT

Thick organic-rich lacustrine sediments of the onshore rift basins of eastern North America constitute a new and potentially important source of hydrocarbons that heretofore have neither been reported nor described in the literature. In the Newark basin of New Jersey, for example, lacustrine deposits comprise a major part of the Triassic-Jurassic stratigraphic section and yield up to 8% organic carbon. Like the Late Cretaceous, nonmarine, producing strata of the Cabinda and Gabon basins of the West African passive margin, these sediments formed along plate margins in wrench-faulted rift basins with deep-water lakes and anoxic bottom conditions. Laterally they interfinger with fan deltas along the border fault and fluvial-deltaic sequences along the tilted margin of the basin. Organic-rich laminated siltstones and carbonates, the result of depressed ecologic efficiency in highly productive lakes, contain (like the Eocene Green River Shale) amorphous algal kerogen, pollen and spores, plant cuticles, and whole fish. einlets and blebs of gilsonitelike deposits locally fill joints and fault surfaces. On the basis of organic maturation and depth of burial, these lacustrine rocks and their contained organics fall within the hydrocarbon window. Having formed in pull-apart basins on the passive margin, these rocks display a variety of structural and stratigraphic traps, and should be given serious consideration in future exploration programs. End_of_Article - Last_Page 1667------------

Subtle Traps in East China Oil-Bearing Basins: ABSTRACT

Some oil-producing basins in eastern China, such as Songliao basin, Bohai Gulf basin, and Nanyang basin are extensively explored regions. In these basins, reserves in structural traps account for 34% of the total proved plus prospective, and 21% of the total estimated; in subtle traps, 7.6% of End_Page 951------------------------------ the proved plus prospective and 38.4% of the estimated. The eastern China oil-bearing basins belong to the intracratonic basins of Mesozoic-Cenozoic age and can be separated into two basic types based on genetic and developing characteristics: depression (Songliao basin) and faulted depression (Bohai Gulf basin). As a symmetrical, gentle syncline, the depressional basin occurred mainly in the Early Cretaceous, forming a unified vast deep-water lake and oil-generating center. Giant deltaic sandstone bodies wedged into the source rocks, constituting a sequence of oil and gas reservoirs. The faulted depressional basin occurred mainly in the Paleogene. The basin is composed of a series of uplifts and wedge-shaped faulted depressions with certain trends. The latter is an independent oil-bearing unit, in which very thick source rocks were deposited. The occurrence and distribution of oil and gas pools are controlled by structural framework and lithofacies zones in the faulted depression. Geologic conditions were excellent for forming various types of subtle oil and gas pools. With the high potential of oil and gas reserves in eastern China, exploration for them will become increasingly important. End_of_Article - Last_Page 952------------

Modification of Membrane Diffusion Chambers for Deep-Water Studies

Membrane diffusion chambers as modified for deep-water lakes provided an improved handling ease, freedom from external damage, extended sampling life, good diffusivity, and excellent statistical reproducibility.

Pleistocene Algal Pinnacles at Searles Lake, California

ABSTRACT Searles Lake is a playa or dry lake which occupies the central floor of Searles Basin, a northwardtrending graben of the Basin and Range province in southeastern California. During pluvial periods of the Pleistocene epoch deep-water lakes filled Searles Basin, and in two of these lakes more than 500 pinnacled masses of calcareous tufa accumulated in an arm-like bay at the southwest end of the basin. Pinnacles are tower-, tombstone-, and cone-shaped masses and large limestone ridges. Most of the pinnacles are 10 to 40 ft high, but a few reach heights between 100 and 140 ft. Basal diameters or widths range from about 10 ft to as much as 500 ft and average 20 to 30 ft. Seven varieties of tufa compose the pinnacles; one of these varieties is also found in lenticular bodies buried in the lacustrine sediments underlying the pinnacles at the northern end of the bay. It is proposed that the pinnacles were precipitated by algae about the orifices of sublacustrine springs issuing along faults striking N. 65° W., N. 50° W., N. 30° E., N. 55° E., and N. 65° E. in the underlying basement rocks. Deposition of the pinnacles at the southwest end of the bay took place in a lake of Tahoe age which filled Searles Basin more than 32,000 years ago. Pinnacles midway along and at the northern end of the bay formed in a lake of Tioga age which initially flooded Searles Basin about 23,000 years ago and lasted until about 10,000 years ago. Tioga Searles Lake at its maximum stand was approximately 460 ft deep and at its highest stage the surface reached an elevation of about 2000 ft.