Source Rock Potential and Oil Source Correlation, Permian (Leonardian) Strata, Central Spraberry Trend, Midland Basin, Texas--Preliminary Study

Edgar H. Guevara, Prasanta K. Mukhopadhyay

Wolfcampian-Leonardian sediments of the Midland basin comprise cyclic sequences of basinal terrigenous and carbonate mudstones overlain by terrigenous submarine-fan deposits. They have not undergone significant uplift, faulting, or water movement. Fractured sandstone and siltstone reservoirs of the Leonardian Spraberry Formation (lower and upper parts), having porosities of as much as 18% but generally less than 10% and permeabilities of less than 1 md, contain most of the 11 billion bbl of oil in the Spraberry/Dean play. About 700 ft (213 m) of basin-plain shales and carbonates, interbedded with basinal turbidite and fan-fringe sequences, separate lower and upper Spraberry reservoirs.

Preliminary geochemical data (total organic carbon, petrography, pyrolysis, and liquid and gas chromatography) from the central Spraberry trend indicate occurrence of (1) mature potential oil-source rocks in the Spraberry Formation and at the base of overlying shales (basal "Clear Fork") and (2) admixtures of terrestrial and marine organic matter, reflecting sediment transport to the basin center. Influence of terrestrial organic matter is greater in lower Spraberry shales (forming type III kerogen) and less important in the basal "Clear Fork," which contains algal, sapropelic IIA kerogen having less hydrogen than do equivalent shallow marine source rocks. N-alkane distribution and biomarker ratios are similar in upper Spraberry oils and rock extracts. However, distribution and ratios in lower Spraberry oils differ from distribution and ratios in upper Spraberry oils and lower and upper Spraberry extracts.

Proximity of mature source rocks to Spraberry reservoirs and geochemical "fingerprinting" suggest (1) multiple oil-source intervals in cyclic Wolfcampian-Leonardian strata, (2) short migration paths between overlapping source and reservoir rocks, and (3) no substantial vertical fluid movement across fractured but thick, impermeable shales that form part of a section unaffected by deep-seated faults.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.