Geochemical Effects and Efficiency of Primary Petroleum Migration in Interbedded Sand/Shale Sequences

D. Leythaeuser

Expulsion of petroleum from mature shale source rocks is not an exhaustive process. Invariably, some of the oil generated remains in the pore system of the source rock. The problems are to determine expulsion efficiency and to understand the type and magnitude of compositional fractionation associated with expulsion. To solve these problems, a mass balance approach was used based on geochemical analysis of closely spaced core samples from interbedded sand/shale sequences. The content and molecular composition of extractable hydrocarbons from the center of thick shale units were compared to those from the edges of the units and from thinly interbedded shales.

The main case histories for this study were two shale/sand sequences of Tertiary and Lower Cretaceous age from Svalbard, Norway, and one of Upper Carboniferous Westfalian age from the Ruhr area, Federal Republic of Germany. In each example, the shales had adequate organic matter contents (Type III kerogens) and maturity levels for hydrocarbon generation, and the interbedded sands had migrated hydrocarbon shows.

Petroleum expulsion is higher from the edges of shale units and from thinly interbedded shales. N-alkane expulsion shows pronounced compositional fractionation with greater expulsion of shorter chain-length molecules. Expulsion efficiencies reach 80-90% at peak generation. Pristane and phytane are expelled to a lesser degree than n-alkane isomers, which increases the pristane/n-C₁₇ ratios in depleted samples. Expulsion efficiencies are not uniform; they vary as a function of proximity to migration avenues, such as sandstones, siltstone lenses, and fractures. Expulsion efficiencies are controlled by source rock richness and the extent of hydrocarbon generation. At equal maturity, Type II kerogen source rocks have higher expulsion efficiencies than Type III source beds.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.