--> Abstract: Hydrocarbons in Miocene Lamproite Dikes of the San Rafael Desert, Utah — Implications for Fault-Controlled Oil Migration and Accumulation in the Western Colorado Plateau, by J. B. Hulen, J. A. Collister, N. F. Dahdan, P. E. Wannamaker, and J. C. Quick; #90937 (1998).

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Abstract: Hydrocarbons in Miocene Lamproite Dikes of the San Rafael Desert, Utah — Implications for Fault-Controlled Oil Migration and Accumulation in the Western Colorado Plateau

HULEN, JEFFREY B., JAMES A. COLLISTER, NICHOLS F. DAHDAH, and PHILLIP E. WANNAMAKER, Energy & Geoscience Institute (EGI) at the University of Utah; and JEFFREY C. QUICK, Utah Geological Survey

Summary

Newly discovered, Miocene-age (22 Ma), vesicular and amygdaloidal lamproite dikes and associated vein and vug-filling minerals in the San Rafael desert of the west-central Colorado Plateau surprisingly contain abundant hydrocarbon. The lamproites and secondary calcite are enriched in solid bitumen; younger chalcedony contains live oil inclusions. The dikes disrupt a broad gentle dome within which their host rock at this level, the Jurassic Entrada Sandstone, is locally and uncharacteristically bleached, and stained with now-degraded oil. Evidence from/geologic mapping, petrographic study, organic geochemistry, and pyrobitumen reflectance suggests that the dike-controlling fractures were the conduits along which oils were introduced into the otherwise oil-barren Entrada. A discontinuous “corridor” of similarly-trending faults extends from the lamproite occurrence gently updip to the huge Elaterite basin tar sand deposit several tens of km to the southeast. This relationship permits speculation that the faults and evidently permeable dikes allowed some of the oil migrating toward the future tar sand to leak upward and charge higher-level traps in the Entrada and other Plateau sandstones. Since the San Rafael desert has been sparingly drilled, such reservoirs may yet await discovery.

Lamproite magmas of this sort are characteristically volatile-rich, extremely overpressured, and capable of creating their own pathways for escape from the mantle lithosphere where they originate. They are even more likely to enhance opportunistically the porosity of existing conduits by natural hydraulic fracturing and phreatomagmatic brecciation. At the San Rafael site, textural relationships among lamproites, allied breccias, and host rocks, when combined with bitumen-reflectance measurements, indicate (1) that the lamproite magmas invaded just such inherited structures; and (2) that the faults had already experienced at least one episode of oil migration. Post-intrusion oil migration is indicated by inclusions of solid bitumen and rare live oil in secondary minerals filling gas vesicles and other cavities in the dike rocks. An important implication of these findings for petroleum exploration in the region is that oil was still available to fill Laramide-age (late Cretaceous to Paleocene) fold traps as late as 22 Ma and probably thereafter.

Sprinkel et al. (1997) have employed organic geochemical and other methods to conclude that the oils of central Utah, including those of the Colorado Plateau, were variously derived from Cretaceous and Permian sources - the Elaterite basin tar sands carry the Permian signature. A Permian source is accordingly more likely for the lamproite-hosted bitumens, since potential Cretaceous sources, now erosionally stripped from the SRD, are regionally hundreds of meters above the Entrada. In-progress organic-geochemical work will allow us to test the validity of this assumption, and at the same time to further constrain the history of oil migration in this sector of the Colorado Plateau.

The San Rafael lamproite occurrence is one of several localities in the Plateau where hydrocarbons are intimately associated with potassic or ultrapotassic intrusive rocks. Ten of twenty-one formerly or currently productive oil and gas fields in the region are encompassed by dike swarms of this composition. The connection could be coincidental, but in view of our work to date on the San Rafael lamproite, it is conceivable that the intrusions in various ways abetted formation of the reservoirs.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah