KATZ, BARRY, Texaco Inc., Houston, TX

ABSTRACT: Controls on Lacustrine Source Rock Development

Although much of the world's petroleum resource base was derived from either marine shales or carbonates, lacustrine source rocks are of volumetric importance in specific geographic regions. The distribution of these source rocks is not only geographically restricted but also appears stratigraphically restricted.

Controlling factors can be divided into three broad categories: (1) controls on the distribution of lake water bodies, (2) controls on productivity, and (3) controls on organic preservation.

The presence of lakes capable of developing important volumes of source rocks requires that the lake be areally significant and long-lived (i.e., stratigraphically persistent). Although a number of mechanisms permit the development of lakes only those of a tectonic origin are capable of developing substantial volumes of source material.

In addition to the presence of a depositional basin, lakes require a positive water balance. Therefore, climate is one of the primary controls on lake distribution.

Climate also influences lake water chemistry and hence the biomass. Water chemistry is also influenced by the character of the rock in the drainage basin. The level of productivity is partially dependent on nutrient availability. Studies have shown that productivity increases with lake maturity suggesting that nutrient recycling is of greater importance that external nutrient supplies. Nutrient recycling is controlled by both climatic factors and lake basin geometry.

Climate and basin geometry also influence organic preservation. Basins with a low width/depth ratio (i.e., rift basins, pull-aparts, etc.) which experience limited seasonal temperature fluctuations display the most favorable characteristics for preservation.

A study of modern lakes suggests that the conditions that favor lacustrine source rock development occur in low altitude, tropical lakes, with water depths between 50 and 400 meters.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.