Abstract: Water-Rock-Organic Matter Interactions at Source (AAPG Short Course: Physical and Chemical Constraints on Migration Theory)

Colin Barker

It is now generally accepted that some of the petroleum generated in organic-rich source rocks can move to reservoir rocks and accumulate. Movement of hydrocarbons (or their precursors) through an aqueous medium involves complex interactions among the mobile organic materials, kerogen, rock matrix, and water. All of these change in composition and/or amount with increasing depth of burial. Water is present in large quantities during the early stages of compaction, but its availability diminishes with depth unless supplied by the smectite-illite conversion. The average pore size in shales also decreases with depth. The organic material available for migration initially contains high concentrations of compounds with oxygen, nitrogen, and sulfur (which impart enhanced solubi ity in water). However, as depth increases these become quantitatively less important, and increased amounts of hydrocarbons are generated from the kerogen. Shallow oils are relatively rich in nitrogen, sulfur, and oxygen compounds but with increasing depths these give way to more mature oils, then condensates, and finally to dry gas. Thus, the migrating material changes with depth as do the rock and water systems through which it moves. Very likely there is not one mechanism of migration by many, and their relative importance depends on local conditions. Some migration mechanisms lead to a preferential movement of certain classes of organic compounds and produce oils with compositions very different from the source-rock extracts. For example, the importance of mechanisms involving the m vement of water out of deep shales will vary widely from one area to another depending on the amount of smectite in the source rock. Because of the varied chemical fractionation it is important to recognize the roles of different migration mechanisms in attempting to correlate oils to their source rocks.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma