Abstract

Turbidite sand in canyon, slope base, fan valley, and depositional lobe deposits interbedded with hemipelagic mud provides an ideal environment for petroleum accumulation. The geometry of sand facies, composition of source mud, and petroleum potential vary with basin size and setting.

In the smallest restricted basins (i.e., carbonate platform troughs < 10 km diameter) with multiple sources of extremely coarse-grained material, channelized turbidite facies do not develop. Thick, coarse-grained sediment gravity flows accumulate rn slope valleys and base-of-slope settings associated with potentially organic-rich slope mud. At greater distances from die slope base, turbidite sand beds become fewer, thinner, and more widely dispersed in basin mud.

Restricted basin fans of intermediate size (i.e., Califomia borderland basins <100 km) fed by canyons intercepting littoral drift cells have excellent petroleum reservoir potential. Low-matrix turbidite sand is channeled to mid-fan lobes and may be interbedded with organic-rich hemipelagic mud. Sandstone continuity extends laterally from inner fan channels to suprafan lobes and vertically within lobe sequences. Tectonic overprint in restricted basin settings commonly permits preservation of turbidite reservoir beds and enhances thermal maturation of organic-rich mud.

Turbidite sand of large, open basin fans fed from major river sources is: (1) finer grained and contains more matrix and (2) channeled to more distal depositional sites than those of restricted basin fans. The organic content of interbedded pelagic or hemipelagic mud is low because of oxidation and infaunal activity. Continuity is poor laterally between lower fan depositional lobes and upper to middle fan channel sand or vertically within mud-rich depositional lobe sand beds. Subduction of deep-sea floor fan complexes may destroy turbidite sand bodies as reservoir sites, just as it may tectonize linear turbidite sand bodies of trench fill and deep-sea channel systems. Similar sand bodies of submarine canyons however may have good petroleum potential where they are enclosed by organic-rich slope mud.

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