Massive Gas- and Oil-Related Carbonates of Louisiana Continental Slope

Harry H. Roberts, Roger Sassen, Paul Aharon, Robert Carney

Recent submersible research on the tops of salt diapirs at the shelf edge (~150 m) and upper slope (~300 m) confirms the occurrence of massive carbonate buildups (to ~20 m relief) in these environmental settings. Samples collected from shallow and deep study sites in the Green Canyon area indicate that the foundation of the massive buildups is authigenic carbonate characterized by isotopically light carbonate carbon, a characteristic that relates their origin to the biodegradation of hydrocarbons. Fault systems associated with the salt diapirs function as conduits for migration of gas and oil to the sea floor. Gas and sediment samples collected from seeps using a submersible indicate that both biogenic and thermogenic gas seepage occurs, as well as crude oil. Bacterial ma s are common to the sea floor of seep areas. Mat samples provided ^dgr¹³C values of -28 to -31^pmil PDB, suggesting an association with crude oil rather than methane.

At water depths greater than about 500 m, gas hydrates form distinctive mounds containing isotopically light carbonate clasts, slabs, and massive rocks in addition to sediments and frozen gas. The chaotic internal structure of shallower massive carbonate mounds (~350-150 m) suggests an origin related to the collapse of hydrate hills and continued carbonate cementation of the components into massive buildups. Through time, vertical movement of salt diapirs and sediment stripping subject hydrates to a shallower environment characterized by an unfavorable temperature and pressure regime. Progressively shallower dome crests at the shelf edge are capped with biogenic carbonates related primarily to low sea level reef-building organisms.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.