Lacustrine siliciclastic rocks and hydrocarbons

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. 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. 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. Although nonmarine reservoir rocks are commonly described as being of a lacustrine origin, many were formedmore » from sediment deposited at the edge of the lake or in settings well removed from the lake. Depositional models of siliclastic 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.« less