Lacustrine Siliciclastic Rocks and Hydrocarbons: ABSTRACT

Siliciclastic rocks formed from sediments deposited in lake basins are source and reservoir units for large accumulations of oil and gas in North and South America, Africa, and Asia. Lacustrine strata of China consist primarily of siliciclastic units; those of Brazil, Angola, and Cabinda are principally of siliciclastic rocks with abundant carbonate beds; those of the United States consist of at least 50% carbonate rock. The sediments were deposited in, or peripheral to, ancient stratified lakes of a variety of ages, which for millions of years maintained a size comparable to that of modern inland seas. In siliciclastic fine-grained beds formed at the depositional center of these large lakes, values of organic carbon commonly average 3% or less. Those sequences of carbonate strata developed in open-lacustrine depositional settings commonly contain greater than 25% organic carbon. However, porosity values are generally very low in reservoir beds associated with the carbonate source rocks (generally cemented with carbonate minerals) relative to those associated with claystone source rocks. The world's largest lacustrine oil and gas fields are developed in depositional systems with very thick sequences of open-lacustrine clay mudstone that contain relatively low values (generally 3% or less) of organic carbon. In these fields, hydrocarbons migrated from a thick central core of claystone into peripheral and overlying lacustrine and alluvial sandstones at relatively shallow burial depths and prior to significant compaction and cementation of the beds. Siliciclastic rocks form the principal reservoir units in most of the world's ancient petroliferous lacustrine depositional systems. Lacustrine turbidite, bar, and deltaic sandstones are important reservoir units in Brazil, Africa, China, and the United States. Although nonmarine reservoir rocks are commonly described as being of a lacustrine origin, many were formed from sediment deposited at the edge of the lake or in settings well removed from the lake. A principal reservoir facies in the Uinta basin, Utah, is composed of siliciclastic beds that developed as the basal parts of coalesced fluvial channels at the fluctuating margin of Paleocene and Eocene Lake Uinta. Oil-bearing strata in some Chinese basins are channel-fill sandstones formed from sediment deposited in moderately sinu us streams on an alluvial plain several kilometers from the lacustrine shoreline. Depositional models of siliciclastic lacustrine rocks constructed to aid in the exploration and exploitation of indigenous hydrocarbon accumulations should be sensitive to type, richness, and thermochemical maturation of organic matter developed and preserved in the lake. Because siliciclastic open-lacustrine beds in many of the large petroliferous lake basins commonly contain relatively small amount of organic matter, thick sequences of such strata may be required to generate large accumulations of oil. Known accumulations are largest where migration has been to porous and permeable beds at relatively shallow burial depths. End_of_Article - Last_Page 462------------