Destruction of a fluvial reservoir by hydrothermal activity (Cameros Basin, Spain)

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Ochoa, M.
Goldstein, Robert H.
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This study provides an example of a high-quality fluvial hydrocarbon reservoir that was completely destroyed by hydrothermal processes. The reservoir unit was deposited in the Cameros Basin, located in the NW sector of the Iberian Chain (Spain). The basin was filled with clastic fluvial deposits (sandstones and conglomerates) between Late Berriasian and Early Aptian times. Provenance of sands was mainly from coarse crystalline rocks. A humid tropical climate produced intense weathering of K-feldspar during transport from source to basin. Thus, a mineralogically mature rigid framework with high porosity existed at the time of deposition, which would have constituted a high-quality hydrocarbon reservoir. At present however, the porosity of the reservoir is negligible. Porosity was reduced by a sequence of diagenetic processes: (1) mechanical compaction (i.e. crushing of metamorphic lithic grains) and chemical compaction, (2) kaolinite and siderite cementation, and (3) early quartz cementation. Hydrocarbon emplacement probably occurred between phases (2) and (3). A low-grade metamorphic (hydrothermal) event, reaching greenschist facies, took place during the Late Cenomanian. It dramatically reduced the remaining porosity of the reservoir and destroyed the hydrocarbon charge. Hydrothermal processes which affected the sandstones include (1) re-compaction; (2) late quartz cementation and silicification of remaining feldspars; (3) carbonate cementation; (4) chloritization of feldspars, metamorphic lithic fragments and intrabasinal argillaceous grains; and (5) growth of pyrite and chloritoid crystals on argillaceous material of intrabasinal, extrabasinal or even diagenetic origin. Hydrocarbons that migrated to the margins of the basin escaped these hydrothermal modifications and were preserved. The results of this study may be used to predict the diagenetic and hydrothermal evolution of other potential reservoirs in similar tectonic settings.
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