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Upper Cretaceous marine strata of the Forbes Formation in the northern Sacramento Valley are composed of terrigenous clastic detritus deposited in westward-prograding shelf to basinal sedimentary environments that existed in a south-trending fore-arc basin. Subsurface stratigraphic investigations in the Bounde Creek gas field indicate that upper, middle, and lower informal divisions of the Forbes Formation are composed of submarine fan and slope sediments deposited in the northern extension of the fore-arc basin.
The upper Forbes contains deposits of inner slope sub-association. Predominantly very fine to fine-grained contourite and feeder channel sands are present within this upper section. These inner slope deposits are underlain by, and are interpreted to pass westwardly into, outer slope and inner fan deposits that comprise the middle part of the Forbes. Sands within this middle part are generally coarser (very fine to coarse-grained), thicker bedded, and have greater stratigraphic irregularity than vertically adjacent deposits within the formation. The lower Forbes consists of middle and outer submarine fan deposits. Sand bodies in this interval were deposited in channels and as suprafan lobes. Ongoing analysis indicates that channel configurations were, in some places, influenced by synd positional faulting that created steep channel margins and abrupt route alterations and/or abandonments. Sand lobe deposits exhibit areal continuity and lateral lithologic gradations.
In the Bounde Creek gas field, distinct hydrocarbon accumulations occur in sand packages that are interpreted to represent: (1) feeder channels and contourites of slope associations; (2) channel lag or mouth bars of middle to inner fan sub-associations; and (3) suprafan lobes of middle to outer submarine fan sub-associations. Paleogeographic reconstructions of depositional horizons within the Forbes reveal a scenario of development and progradation of these inner slope to outer fan associations, and allow geometric projections of hydrocarbon-bearing sand bodies within the system.
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