Rodessa Formation, Bossier Parish, Louisiana: Lithofacies Analysis of Hydrocarbon-Productive Shallow-Water Clastic/Carbonate Sequence

Jamsie L. Roberts, Brian E. Lock

Sligo field in Bossier Parish has produced more than 2 tcf of natural gas, 25 million bbl of oil, and 8.5 million bbl of condensate since its discovery in the 1920s. Although the field is in a mature stage of development, potential still exists for extensions to deeper pools and into flank areas. Production is from a large number of stratigraphic horizons. The northwestern quarter of the field underlies Barksdale Air Force Base Federal Reserve and cannot be regarded as fully developed.

Cores taken from the upper parts of the Rodessa in Sligo field consist of a basal sand overlain by carbonates. The sand contains abundant examples of distinctive tidal sedimentary structures and is interpreted as a record of high-energy tidal sand-flat deposition. The carbonates are shallow-water wackestones, packstones, and rare grainstones, some with monopleurid rudists in growth position, indicating shallow, relatively low-energy restricted (possibly slightly brackish) lagoonal conditions. Oysters occur abundantly in certain lithofacies and again suggest somewhat brackish conditions. Scattered ooliths are probably a result of storms washing in debris from higher energy shoals seaward.

Temporary exposure of the sea floor is suggested by the presence of widely traceable diastems with oysters cemented to the eroded surfaces and Lithophagus borings into the substrate. Most significantly, the underlying few feet of strata have been dolomitized and provide the main productive horizons in the wells studied. Supratidal evaporites confirm that sedimentation occurred close to an arid shoreline (perhaps the margin of the Sabine Uplift island or peninsula) with possible periodic influx of fresh waters.

The sedimentology of the Rodessa is an important component of the petroleum geology and provides an understanding of the reservoir rock, which should aid in further exploration in northern Louisiana.

AAPG Search and Discovery Article #91036©1988 GCAGS and SEPM Gulf Coast Section Meeting; New Orleans, Louisiana, 19-21 October 1988.