Modern Lacustrine Deltas: Exploration and Production Analogs

Robert S. Tye, James M. Coleman, Ray E. Ferrell, Jr.

Lacustrine deltas represent (100 km² by 3-5 m thick) sand-rich sedimentary accumulations formed during the alluviation of interdistributary basins. Basin subsidence and a variable supply of sandy sediment result in stacked delta lobes separated by fine-grained backswamp deposits.

In the 30-m topstratum of the Atchafalaya basin, Louisiana, three to four lacustrine deltas are preserved, each bounded by rooted backswamp clay. The deltas form a coarsening-upward sequence of basal laminated to burrowed prodelta clay gradationally overlain by rippled delta front sediments which grade into rippled and cross-laminated, medium-grained, distributary-mouth bar deposits. Sandstone geometry is controlled by river-mouth processes, basin shape, and bathymetry. Thickest accumulations occur in linear, dipelongate lobes. Mud-filled channels and interdistributary troughs disrupt sand continuity along strike.

Potential reservoirs consist of multiple lacustrine deltaic sandstones interbedded with backswamp shale. Communication between sandstones can be provided by the lateral occurrence of a thick meander-belt sandstone. Fluids within each deltaic reservoir will preferentially flow along depositional dip. Their association with shales formed in abandoned channels, interdistributary troughs, and prodelta facies increases reservoir heterogeneity. Precipitation of carbonate, sulfide, and clay cements during early diagenesis is common. Dominant clay minerals are smectite and hydroxy-interlayered vermiculite, or degraded illite. Burial-induced changes in these affect fluid movement and reservoir porosity.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.