Major Hydrocarbon Source Rocks Research Articles

Benthic community analysis and the prediction of organic carbon content in Mesozoic black shale facies

The burial and preservation of significant quantities of marine organic matter in epicontinental basins results from low bottom water oxygen levels, and moderate to low sedimentation rates of fine-grained siliciclastic sediment under at least moderately productive waters. Because benthic biotas are extremely sensitive to such factors as bottom water oxygen, grain size, substrate consistency and sedimentation rate, there is a potentially predictive relationship between biofacies and Corg potential of marine strata. High-resolution biological, sedimentological and geochemical studies of Mesozoic organic-rich black shale facies, comprising major hydrocarbon source rocks, allow characterization of this relationship: a) A series of recurrent, bivalve-dominated benthic communities are defined based on trends in diversity, abundance, equitability and trophic specialization. They comprise biofacies which reflect oxygen and substrate gradients in time and space, and thus the Corg preservation potential of marine sediments/strata. The biotas are surprisingly robust in terms of population size and inferred biomass, and demonstrably colonized the benthic zone (as opposed to pseudoplanktic origins). They include both resident communities of taxa specifically adapted to low oxygen (dysoxic) conditions through physiological and anatomical means (e.g., expanded oxygen adsorptive surfaces), or possibly through bacterial chemosymbiosis, and event communities of less tolerant taxa reflecting short-term improvements in benthic conditions; b) many organic-rich sequences contain common event biofacies, indicating a dynamic benthic environment with frequent short-term fluctuations in oxygen and substrate. These are attributable to mixing during large storms, productivity blooms, rapid deposition from mass flows, or reduced sedimentation during condensation/bypass events. However, increasing frequency of events is not necessarily correlated with decrease in Corg levels; c) comparison of trends in the quantity and quality of Corg to biofacies patterns indicate that the most favorable Corg preservation is associated with “benthic boundary biofacies.” These biofacies represent a redox boundary at or near the sediment-water interface and a predominantly dysoxic to episodically oxic water column. The “benthic boundary” may have been associated with the presence of microbial (mainly sulphur bacterial) mats on the substrate surface, which could have regulated its position and contributed to the pool of sedimented Corg; d) mapping of benthic boundary biofacies in time and space within a basin makes possible correlation of Corg preservation potential to paleogeography and sea level history. Although the results of such analyses from the Greenhorn Formation, Western Interior basin, suggest highest Corg burial and preservation in fine-grained shale facies of distal offshore environments during sea level rise, high Corg is not necessarily associated with an anoxic water column. This suggests the need for reappraisal of stagnant basin scenarios for source rock formation. The most important factors in Corg accumulation and preservation may be the development of benthic boundaries and consequent maintenance of anoxia below the sediment-water interface in fine-grained, organic-rich substrates, and the contribution of bacteria to the flux of Corg into the sediment.

Depositional cycles and source rock development in an epeiric intra-platform basin: the Hanifa Formation of the Arabian peninsula

The Upper Oxfordian to Lower Kimmeridgian Hanifa Formation was deposited in an intra-platform basin on the broad epeiric shallow-water carbonate platform that extended over much of the Middle East during the Mesozoic. The formation consists of organic-rich carbonate mudstones, bioturbated mudstones and sulphates. The organic-rich mudstones are considered to be one of the major hydrocarbon source rocks in the Middle East oil province. The correlation of sedimentary facies shows a hierarchical stacking pattern of transgressive-regressive depositional cycles within the Hanifa Formation. Since these cycles occur at a basin-wide scale, it is assumed that they are related to relative changes of sea level. The superimposed scales of cyclicity determine the occurrence of source rock and sulphates. Major source rock development occurs in transgressive-dominated cycles. Sulphates, on the other hand, are more common during periods of overall regression.

MATURITY OF ORGANIC MAITER IN THE JURASSIC OF SOUTHERN ENGLAND AND ITS RELATION TO THE BURIAL HISTORY OF THE SEDIMENTS

An investigation of the maturity levels of organic matter in the major potential Jurassic hydrocarbon source rock units in Southern England (i.e. the Lower Lias, Oxford Clay and Kimmeridge Clay) using sporomorph colouration and alkane distribution patterns shows that the Oxford Clay and the Kimmeridge Clay organic materials are immature in most parts of the area. They are mature only in the central parts of the Mesozoic Wealden Basin.The distribution of mature Lower Lias sections is found to be more extensive. Attempted reconstruction of the burial histories of Jurassic sections across Southern England, and the application of the Lopatin method of theoretical organic maturity prediction, seem to support the observed maturity trends and suggest that the present maturity levels of Jurassic organic matter at any given locality are dependent on the structural development of that area through time.

Undrilled Shallow Giant Trap in Denver Basin, Colorado: Mountain-Front Thrust: ABSTRACT

Along the southwestern margin of the Denver basin, Precambrian rocks have been upthrusted at least 15,000 ft (4,600 m) in the Front Range and 8,000 to 10,000 ft (2,400 to 3,000 m) or more in the Wet Mountains. Strongest deformation apparently was during latest Cretaceous, Paleocene, and possibly earliest Eocene. The sedimentary strata of the basin were folded upward and were overridden by the Precambrian to create the steep west flank of the basin. Below the Precambrian, the precise configuration of the strata and the faults is unknown because there are no available seismic or drilling data, but reasonable interpretations can be made by analogy with other similar areas. Important reservoirs in the basin are the Permian Lyons Sandstone, the Lower Cretaceous J and D sandstones, and the Upper Cretaceous Codell Sandstone, Niobrara Formation, and Pierre Shale. The Lyons Sandstone, largely eolian, is about 700 ft (210 m) thick on outcrop at Colorado Springs in front of the upthrust. The J sandstone is thicker in drill holes directly in front of the upthrust than almost anywhere else in the basin. Distributary complexes in the J form clastic wedges almost 200 ft (60 m) thick that extend eastward from the mountains. The wedges provide migration paths that can funnel hydrocarbons from the basin upward into traps in the wedges themselves below the Precambrian. Deposits of southwest-oriented barrier islands and lagoons in the 40-ft (12 m) thick Codell S ndstone trend under the upthrust of the Wet Mountains. Figure Directly overlying the J are the major hydrocarbon-source rocks in the basin (in ascending order: the Graneros Shale, Greenhorn Limestone, Carlile Shale, and Niobrara Formation). Black shale is interstratified with the Lyons Sandstone in at least one drill hole in front of the upthrust. All source rocks probably reached maturity in late Cretaceous time and still are generating today. Oil and gas fields are present in front of the Front Range and Wet Mountains, and oil seeps are present at each end of the Front Range mountain-front thrust system. Below the Precambrian, simple upfolding permits an oil column as much as 5,000 ft (1,500 m) high, or more, in the J, and as much as 4,000 ft (1,200 m) high, or more, in the Lyons, assuming a fault dip of 70° at depth; lower fault dips permit higher oil columns. Clayey fault gouge, breccia, and minute faulting, in a zone that is in many places hundreds of feet wide at the fault, should be a good hydrocarbon seal, like a cork in a tilted 5,000-ft (1,500 m) high bottle. If the strata to the west to form a large anticline below the Precambrian, a different kind of trap of very large dimensions would be present. Any kind of trap can extend a combined north-south distance of nearly 65 mi (105 km). Even if roll over is absent and the fault dips steeply, drilling depths to ost traps are likely to be only several thousand feet. End_of_Article - Last_Page 488------------