ABSTRACT: Interior Cratonic Sag Basins
Morris W. Leighton
The following summary has been developed from the Cratonic Basins volume, one of the volumes on major sedimentary basin types to be published by the AAPG as one of the World Petroleum Basin Memoirs.
Cratonic basins are thickened, somewhat symmetrically shaped accumulations of sediment located entirely on continental crust. Generally more or less circular or oval in shape, the basins are located in the interiors of continental masses. Some are single-cycle basins containing sediments deposited during one tectonic cycle, and others contain repeated sag cycles or are complex multicycle basins. A number of them appear to have developed initially along and ultimately over ancient rift systems. Interior cratonic sag basins are typified by ancient continental crust that has subsided episodically over spans of 10-100 m.y. or more. The basins are characterized by a dominance of flexural over fault-controlled subsidence, and the ratio of the volume of sediments to the surface area of sag b sins is low.
For the AAPG volume, the Illinois, Michigan, Williston, Baltic, and Paris basins were selected as petroleum productive analogs of cratonic basins, and the Parana and Carpentaria basins were chosen as nonproductive analogs. The Parana basin, the largest of these basins, covers 541,000 mi2, and the smallest, the Paris basin, 43,000 mi2. The thickest section of sediments (+23,000 ft) is found in the Illinois basin, the thinnest (6600 ft) in the Carpentaria basin. Of these basins mentioned, all but the Paris basin, which developed on Hercynian basement, formed on Precambrian rocks. The Illinois, Michigan, and Baltic basins consist mainly of Paleozoic sequences, whereas the Paris and Carpentaria basins contain Mesozoic sediments. The Parana and Williston basins contai thick sequences of both Paleozoic and Mesozoic rocks.
Various theories have been proposed to explain the mechanisms involved in the sagging of the continental crust to create the space required for the volume of sediments characteristic of this type of basin. The mechanisms remain unclear; the two most popular explanations at the present time are thermal subsidence and/or isostatically uncompensated mass emplaced in the lower crest prior to basin development.
Extrinsic forces accompanying plate tectonics and paleoclimatic fluctuations appear to have exerted significant influence during basin development. It is suggested that cratonic basins, such as the Illinois basin, adjacent, but at an angle, to an ancient cratonic plate margin, contain the thickest sediments; apparently subsidence rates of these basins were greater than those of cratonic basins that formed farther inboard on the interior of continental plates, i.e., than those more remote from the plate edges. Those cratonic basins positioned farther inboard of plate margins, well on the interior parts of continental plates, and in low paleolatitudes (e.g., the Williston, Michigan, and Baltic basins) are more likely to contain evaporites; these locations provided greater opportunities or restricted marine circulation accompanied by long periods of evaporation. The history of interior cratonic sag basins also has been influenced strongly by the subsequent positions assumed by these basins on continental plates. The Parana basin, for example, which initially developed well away from plate edges, continued as a sag basin at or near the margin of the South America plate following the split from Africa in the Mesozoic; however, close proximity to the breakup margin led to the intrusion of thick diabase sills and dikes and extrusion of large volumes volcanic rocks. These volcanic features and the processes that accompanied them created major complexities in the development of hydrocarbon plays as well as in their mapability.
Analysis of the basins reveals that tectonics played an important role in controlling the shapes and juxtapositions of the packets of sedimentary sequences; sea level changes appear to have had a major influence on sedimentation patterns within each sequence.
Sag basins situated in low latitudes during their development commonly were the sites of deposition of thick carbonates (e.g., the Illinois, Michigan, and Williston basins during the Paleozoic, and the Paris basin during the Mesozoic). However, siliciclastic sedimentation characterized basins formed in higher latitudes (e.g., the Parana and Carpentaria basins). Paleoclimatic control on the type of sedimentation is thus indicated.
In 1980, Huff estimated that cratonic and foreland basins together contain 1200 billion bbl of oil and gas equivalent, constituting a very large part of the Free World's reserves. These basins have yielded a substantial number of giant oil and gas fields. Of the 799 sedimentary provinces, listed by St. John et al. in 1984, between 9 and 10% are cratonic basins. On the average, only 8% of the simple cratonic basins contain giant fields--those yielding more than 500 million bbl of recoverable oil; however, about 26% of cratonic basins underlain by rifts contain giant fields. The largest oil and gas fields appear to be related to basement-involved structures, either anticlines or faulted structures. Numerous subtle traps with significant stratigraphic trapping components also exist, and oth vertical and long-distance migrations have been effective in filling these traps in interior cratonic sag basin settings.
Highly productive interior cratonic sag basins are characterized by widespread, thick, mature, organic-rich source rocks, large structures, carbonate or siliciclastic reservoirs, and adequate seals. Nonproductive basins have one or more of the following characteristics: immature source rocks, a leaky plumbing system, freshwater flushing, and/or complex geology due to numerous intrusions that produce anomalous maturation patterns and inhibit mapability.
New play concepts can aid in exploration for new reserves, and new tricks of the trade, including horizontal drilling, recognition and definition of reservoir heterogeneities and improved well spacing, improved well completion, and stimulation practices can increase production from even the most mature of cratonic basins.
AAPG Search and Discovery Article #91001©1989-1990 AAPG Distinguished Lecture Tours 1989-1990