Loma Chumico Shale: A super-rich source rock with unusual geochemical characteristics
- Mobil Research and Development Corporation, Dallas, TX (United States)
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 (approx. 480 m) were analyzed. 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-1 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 C[sub 25] tail-to-tail isoprenoid and novel C[sub 27] and C[sub 28] isoprenoid hydrocarbons with pentacyclic rings. These compounds are believed to be derived from thermophilic 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-1 well could generate more than three barrels of oil/m[sup 3](approximately 4000 bbl/ac-ft-ft). The oil would be heavy and sulfur-rich.
- OSTI ID:
- 6104295
- Report Number(s):
- CONF-930306-; CODEN: AABUD2
- Journal Information:
- AAPG Bulletin (American Association of Petroleum Geologists); (United States), Vol. 77:2; Conference: International congress of the American Association of Petroleum Geologists (AAPG), Caracas (Venezuela), 14-17 Mar 1993; ISSN 0149-1423
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
COSTA RICA
RESOURCE POTENTIAL
ORGANIC MATTER
EXTRACTION
SHALES
GEOCHEMISTRY
ALKANES
BIOLOGICAL MARKERS
DEPOSITION
HYDROCARBONS
KEROGEN
PETROLEUM
POLAR COMPOUNDS
PYROLYSIS
SEDIMENTARY ROCKS
SOURCE ROCKS
BITUMINOUS MATERIALS
CARBONACEOUS MATERIALS
CENTRAL AMERICA
CHEMICAL REACTIONS
CHEMISTRY
DECOMPOSITION
DEVELOPING COUNTRIES
ENERGY SOURCES
FOSSIL FUELS
FUELS
LATIN AMERICA
MATERIALS
MATTER
ORGANIC COMPOUNDS
ROCKS
SEPARATION PROCESSES
THERMOCHEMICAL PROCESSES
020200* - Petroleum- Reserves
Geology
& Exploration