The Forbes Formation (Santonian and lower Campanian) and Winters Formation (upper Campanian and lower Maastrichtian) are highly productive turbidite reservoirs in the Sacramento forearc basin of northern California. However, their reservoir geometries and architectures differ markedly, chiefly as a result of differences in sediment supply and orientation of sediment input into the basin. The Forbes Formation has been penetrated by thousands of wells and is productive in about 50 gas fields. It consists, in ascending stratigraphic order, of four major architectural elements that can be distinguished in well logs, seismic-reflection data, and outcrop: (1) a basal massive, concretionary basinwide mudstone, the Dobbins Shale Member, interpreted to be a condensed section deposited during a sea-level highstand; (2) an interval of shale with abundant laterally continuous thin-bedded and fine-grained turbidites interpreted to be a basin-plain succession; (3) a thick interval of mudstone with interbedded sandy and silty turbidites, interpreted to be a mud-rich submarine fan dominated by meandering channel-levee complexes that form the principal reservoir units; and (4) an upper interval of mostly mudstone with laterally discontinuous sandstone bodies incised into south-sloping clinoforms, interpreted to be a muddy slope succession containing abundant sandstone-filled gullies. The deep-marine system was fed by a south-prograding fluvial-dominated delta, the Kione Formation. Both units are overlain by a second basinwide massive mudstone, the Sacramento Shale. The meanderform, ribbon-shaped channel-levee complexes, oriented subparallel to the structural axis of the basin, produce from a variety of traps. The overlying Winters Formation in the central and southern Sacramento basin has also been penetrated by thousands of wells, and produces from about 40 gas fields. It overlies the Sacramento Shale, interpreted to have been deposited as a condensed section during a long sea-level highstand. The Winters Formation consists, in ascending stratigraphic order, of (1) an interval of mostly shale interpreted to be a basin-plain succession, (2) a complex of pod-shaped sandstone bodies interpreted to have been deposited as sand-rich submarine fans dominated by suprafan-type sandstone bodies that form the principal reservoir units, and (3) muddy slope deposits that contain southwest-dipping clinoforms incised by prominent sandstone-filled submarine canyons. The deep-marine system was fed by a complex of west- and southwest-prograding wave-dominated deltas of the Starkey Formation. The small, pod-shaped submarine fans and pod-shaped suprafan sandstone bodies produce from a variety of traps, and provide a model for other sand-rich, delta-fed submarine-fan systems deposited in restricted basins with modest amounts of shelf accommodation space.
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