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WALTERS, CLIFFORD C., Mobil Research and Development Corporation, Dallas, TX, STEVE E. PIERCE, Mobil New Exploration Ventures, JAMES R. GORMLY, Mobil Exploration Nor., and MELODYE A. ROONEY, Mobil Research and Development Corporation, Dallas, TX

ABSTRACT: Loma Chumico Shale: A Super-Rich Source Rock with Unusual Geochemical Characteristics

The Loma Chumico Shales occur in the Late Cretaceous ophiolitic Nicoya Complex in western Costa Rica. The shales are included in the sedimentary part of the complex that overlies igneous pillow basalts, volcanic agglomerates with interbedded sedimentary rocks, and intrusives. Samples of Loma Chumico Shale (approximately 480 m) were analyzed from the Morote-1 well. This well was drilled by RECOPE, the National Oil Company of Costa Rica, in 1986 and reached a total depth of 631.5 m.

The shales are exceptionally rich in organic matter (24 to 29% TOC) and contain kerogen that is rich in hydrogen (Hydrogen Indices = approximately 800 mg hydrocarbons/g of rock, HIC = 1.397) and sulfur (9.7% S, S/C=0.048). The Loma Chumico Shales in the Morote-I well are immature. This is supported by petrographic, Rock-Eval (Tmax approximately 415 degrees C), and biomarker analyses. Nevertheless, the shales have a high concentration of extractable organic matter (EOM approximately 30,000 ppm) and suggest that early oil generation has occurred. Saturated hydrocarbons account for less than 8% of the EOM and are predominantly composed of a C25 tail-to-tail isoprenoid and novel C27 and C28 isoprenoid hydrocarbons with pentacyclic rings. These compounds are believed to be derived from the mophilic archaebacteria. The saturated biomarkers form an incomplete picture of the depositional setting as many compounds are sulfur-sequestered; however, the presence of thermophilic archaebacteria suggests that deposition occurred in a hydrothermal environment. Pyrolysis and chemical degradation studies of kerogen and polar compounds liberate a more typical distribution of n-alkanes, isoprenoids, and biomarker compounds.

The Loma Chumico Shales could be a major source unit for petroleum in Costa Rica if the super-rich facies has a wide areal extent and the shales obtain sufficient thermal maturity. The Loma Chumico Shales in the Morote-l well could generate more than three barrels of oil/cu m (approximately 4000 bbl/ac-ft-ft). The oil would be heavy and sulfur-rich.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.