Terrigenous Clastic Sediments Research Articles

New Look at Geology and Petroleum Potential of Northern Alaska: ABSTRACT

Three terranes in northern Alaska have potential for petroleum: a Tertiary basin in the eastern Arctic Plain, a post-Neocomian Cretaceous basin in the Northern Foothills and western Arctic Plain, and a complex of thrust-faulted late Paleozoic and pre-Albian Mesozoic rocks in the Brooks Range and Southern Foothills. Recent re-analysis of these terranes suggests that the disposition of extensive thrust sheets may have controlled the distribution of petroleum reservoirs in much of the area. This interpretation has significant implications in evaluating the petroleum potential of northern Alaska. The Tertiary basin contains interfingering marine and non-marine clastics. A few open folds are present and may provide structural traps. Stratigraphic traps may be expected along the tectonically active southern margin and along the stable basement rise under the present continental shelf. Elements of the Brooks Range may have been thrust over the southern margin of the basin in late Pliocene time. This basin awaits exploratory drilling. In the post-Neocomian Cretaceous basin, interfingering marine and non-marine terrigenous clastic sediments in open folds offer a host of structural and stratigraphic traps. Some of these have been drilled, and a few contain sizable reserves of high-quality oil and gas. Stratigraphic traps may be expected also in pre-Cretaceous rocks along the basemen rise that forms the northern margin of the basin. Buried detachment fault planes may underlie some of the southern folds, offering the possibility of different, and possibly equally interesting, structures and stratigraphic sections. The Brooks Range and Southern Foothills terrane is the most complex and difficult to assess, but its geology and oil potential are the most intriguing. The distribution of formations and facies is the result of northward movement of extensive thrust sheets during at least two major episodes of thrusting in mid-Early Cretaceous (pre-Albian) and early Tertiary times. Tectonic movement may have been as much as 75 miles, thus telescoping facies trends in all formations. As a result, Upper Devonian through Lower Cretaceous (Neocomian) rocks of numerous facies are now exposed in a belt of imbricate thrust plates. Holes drilled in this terrane can test a variety of structures and numerous facies of several formations, but the geology is so complex that paleogeographical and palinspastic reco structions must precede the drilling. Similar terranes elsewhere have been very productive. End_of_Article - Last_Page 622------------

Stratigraphy and Structure of Parras Basin and Adjacent Areas of Northeastern Mexico: ABSTRACT

The Parras basin, in southern Coahuila and western Nuevo Leon, contains 15,000-20,000 feet of Upper Cretaceous and lower Tertiary terrigenous clastic sediments. From 5,000-7,000 feet of Lower Cretaceous carbonate rocks and 6,000-10,000 feet of Triassic and (or) Jurassic evaporites, carbonate, and terrigenous clastic rocks flank parts of the basin and underlie large areas within the basin. The Triassic and (or) Jurassic sedimentary rocks exhibit complex facies relations. Lower Cretaceous carbonate rocks are remarkably uniform over large areas of northeastern Mexico. Most of the Upper Cretaceous and lower Tertiary calcareous-arenaceous-argillaceous sediments were deposited in a boot-shaped, shallow, subsiding basin between the present-day Sierra Madre Oriental and the Coahu la Platform. The Upper Cretaceous-lower Tertiary Difunta Group displays intertonguing relations between two distinct lithic types; red, non-marine, arenaceous-argillaceous End_Page 639------------------------------ strata alternate with gray to black, brackish, calcareous-arenaceous-argillaceous strata. The red formations pinch out or change facies toward the north and east in the basin. Marine deposition was continuous from Late Cretaceous to early Tertiary in the eastern part of the basin. A few redbeds grade down depositional dip into gray, marine strata. Red strata have been discovered in the Upper Cretaceous Parras Shale, which normally is a gray to black, calcareous shale, 4,000-5,000 feet thick. During Paleocene or Eocene time, the sediments of the Parras basin were deformed contemporaneously with the adjacent Sierra Madre Oriental. Deformational intensity in the Lower Cretaceous carbonate rocks of the Sierra Madre appears related to the distribution and thickness of the Minas Viejas (Jurassic?) evaporites. The type and degree of deformation in the Upper Cretaceous Parras basin is not uniform as indicated by the following: (1) overturned folds and imbricate thrusts, which probably do not extend below the Parras Shale, characterize the constricted western part of the basin; (2) broad, elongate, open folds in the southeastern part extend downward to folds in Lower Cretaceous strata; and (3) broad, open, domal folds in the northeast are related to Lower Cretaceous uplifts on the surface and at depth. End_of_Article - Last_Page 640------------

Stratigraphic Analysis of Source-Bed Occurrences and Reservoir Oil Gravities

A knowledge of hydrocarbon source-bed occurrences and lithologic types is fundamental to an understanding of the distribution of oil accumulations. A study of the relations between source-bed lithology and the crude oil in producing reservoirs in the southeastern United States and West Texas was undertaken to provide geologic data on factors affecting oil gravities and the time and direction of migration. This study shows that distinct oil-gravity characteristics can be correlated with source-bed lithologic types, and that the depth of burial which is required to convert oil to gas condensate also influences the oil-gravity characteristics. In addition, the study shows that an approximate overburden of 2,000 feet is needed for flush migration from the source bed. Statisti al analyses, theory, and major worldwide oil-field examples verify the dominant role of shale, marl, and clay as source beds for the underlying reservoirs. In contrast, shale-free carbonate sequences contain oil accumulations whose source is an underlying shaly, fine-textured, or phosphatic limestone. Crude oil in the Ellenburger of West Texas originated in shale source beds in the Simpson, Woodford, and Lower Pennsylvanian, each of which exhibits a distinctive curve showing the variations between oil gravity and depth. Other primary source-bed sequences in the Permian basin are Pennsylvanian dark gray shale, Wolfcampian and Leonardian basinal terrigenous clastic sediments (dark shale and siltstone), and Guadalupian basinal and lagoonal terrigenous clastics.

Nearshore Depositional Environments of Upper Cretaceous Panther Tongue, East-Central Utah: ABSTRACT

The Upper Cretaceous Panther Tongue of east-central Utah is exceptionally well exposed in the Book Cliffs and Wasatch plateau and offers an unusual opportunity to study, in three dimensions, the morphology and internal structure of a nearshore depositional complex. Lithology, sedimentary structures, geometry, and trace fossil assemblages, considered collectively, permit recognition of seven depositional environments during Panther time. Transition from the underlying Mancos Shale to the coarser terrigenous clastic sediments of the Panther Tongue is represented by densely mottled gray siltstone in which primary lamination has been destroyed by the activity of detritus-feeding organisms. Overlying the gray siltstone is a very fine-grained sandstone which has a high matrix content and contains an abundant and varied trace fossil assemblage. This very fine-grained sandstone is present in bar and backbar depositional environments, but is divided into two areas of occurrence by an elongate sandstone body believed to be a longshore bar. The wedge-shaped forebar contains wavy laminated sediments near the base which grade upward into thin-bedded mottled sediments. Trace faunas in the forebar change in nature vertically and la erally away from the longshore bar. In the backbar, sediments are more poorly sorted and stratification is thin- to thick-bedded. The trace fossil assemblage in the backbar shows very little lateral change and is less variable than in the forebar. The longshore bar strikes northeast-southwest, and its cross section is well exposed in the western Book Cliffs and northern Wasatch plateau. The bar is characterized by massive bedding, lenticular shape, and asymmetrical flanks. Large-scale, low-angle (10°) foreset beds at the Panther type locality north of Helper, Utah, represent the seaward (southeast) face of the bar. These foreset beds have ripple-marked surfaces and contain an abundant and varied suite of sole marks. The dip and predominant current direction of sole marks are southwest, indicating that longshore currents flowed along the seaward side of the bar. Current ripples and oscillation ripple marks, however, developed in response to tidal action, as is indicated by orientation of their crests parallel with the longs ore bar. The bar and bar shore-face contain a characteristic trace fossil assemblage which is dominated by filter-feeding organisms and vagrants which left various surface trails on bedding planes. Behind the bar, a series of short, low- to medium-angle (5-20°), cross-stratified sandstone beds built landward over the backbar environment. A marine transgression late in Panther time truncated the previously deposited sediments, and a sequence of horizontal to subhorizontal strata was laid down across the erosional surface during a second regression of the sea. Panther deposition ended with the return of the Mancos sea and the deposition of clay and mud. End_of_Article - Last_Page 618------------

Stratigraphic Models in Exploration

The S. E. P. M. [Society of Economic Paleontologists and Mineralogists] is tending to become the stratigraphic arm of exploration geology and its greatest contributions have been made through integration of individual specialties into the broad field of stratigraphy. One small part of this integration is to consider stratigraphic models which express, in a systematic form, the interaction of a defined set of variables which operate to produce an observable result. The variables are geological processes and the result is a sedimentary response; consideration is limited to terrigenous clastic sediments deposited at or near marine strand lines. The shape of a body of sedimentary rocks varies as a function of the quantity of material supplied to the depositional site, the rate of subsidence at the site, the rate of dispersal, and the nature of the material supplied. Stratigraphic models based upon these variables are shown for cratonic sources with equal subsidence for regression and transgression, linear subsidence, nonlinear subsidence, and external sources producing clastic wedges and turbidites.