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Tectonic and Geomorphic Controls on Cyclic Lacustrine and Fluvial Deposition

Abstract

New Aptian and Albian pre-salt reservoirs and sources in South Atlantic margin basins underscore the importance of lacustrine systems in the generation and trapping of hydrocarbons. However, there are few outcrop analogs that reveal in detail the distribution of facies and their relationship to structures within similar lacustrine basins. Aptian strata of West Texas include a thick (up to 1 km) section of lacustrine beds, separated by thin fluvial intervals. The exposures cover an area of at least 1,700 km, however, the margins are not exposed and the lake may have been much larger. In the Indio Mountains of West Texas, thrusts have juxtaposed three 6-9 km long exposures, each parallel to the Aptian rift margin and originally spaced 4 and 12 km apart. This provides a 3-dimensional exposure of the lacustrine stratigraphy. Carbonate nodule-rich shales with discontinuous limestones dominate the lacustrine sediments. Shales contain ostracods, gastropods and charophyte algae and contain widely dispersed, but thin lenses of microbial limestone. The shale and limestone intervals are separated by channelized sandstones and conglomerates. Key features relevant to petroleum reservoirs and sources are: 1) The lake probably initiated in the deeper basins of the rift and onlapped onto intra-basinal highs, 2) However, the wide spread shales and interstratified incised fluvial sandstones in all fault panels require a lake with a broad, relatively flat bottom that infilled rift basins as the lake expanded, 3) Cyclic deposition reflects repeated incursions by a fluvially-dominated delta into a, shallow, low relief lake, resulting in cyclical shallowing-upward, parasequence-like architecture. Most fluvial deposition was from an axial stream that lay to the southeast. However, monomict conglomerate channel fills must have been deposited from sources from nearby rift flank uplifts, 4) Channels are concentrated within 1 – 4km wide, channel-rich belts oblique to basin margins and did not shift throughout deposition of the formation. This requires structural localization of the axial drainages, probably by transfer zones separating rift segments.