Grain-Size Analysis of the Distal Mississippi Fan: Insights into Flow Processes, Facies Distribution and Reservoir Quality Prediction

Fildani, Andrea; Clark, Julian; Power, Bruce; Sullivan, Morgan; and Covault, Jacob
[email protected]

The Mississippi Fan blankets the modern seafloor of the Gulf of Mexico. This submarine fan system has been studied since the late 1960s, resulting in an extensive database of geophysical surveys and samples. In 1983, Leg 96 of the Deep Sea Drilling Project (DSDP) drilled and collected core from the Mississippi Fan at nine sites from the recent channel, levee and lobes (Bouma et al., 1986). We described and sampled sediment cores from distal lobe Sites 614 and 615 and proximal channel Site 621 in order to characterize distal, sheet facies and analog reservoir quality.

Sites 614 and 615 are in the distal-most part of the Mississippi fan, approximately 400 km from the shelf and approximately 100 km downdip of the channel-to-lobe transition. The ~200 m thick cores are characterized by thick- (>0.5 m) and thin- (<0.2 m) bedded sandy facies. We interpret these sediments to comprise a progradational sheet complex-set, several hundred feet thick, consisting of a basal-most distal outer sheet complex, an inner/middle-sheet complex, and an upper-most aggradational channel-levee complex.

Samples from the Mississippi Fan cores allow a quantitative analysis of grain-size distribution before any significant diagenetic modification, which can be correlated to interpreted depositional facies. Preliminary analysis suggests massive and structure-less “sandstones” with high fine-grained content cannot be described as Ta Bouma division turbidites. Instead, they are silty sandstone with relatively poor reservoir quality representing transitional facies between turbidites and debrites and need to be treated separately from the cleaner turbidite deposits in forward modeling of reservoir quality. This can serve as a useful analog for interpretation of hydrocarbon reservoirs interpreted to be deposited in similar environments (such as the Wilcox). To explore and assess differences in reservoir quality among different facies, we obtained 168 sand plug samples for quantitative grain size (laser particle) analysis.

Hydrocarbon reservoir quality of sediment deposited in basin floor fan environments is affected by the detrital grain size of the sediment. Sediments interpreted to have been deposited in lobe environments show a mix of facies interpreted as clean turbidite sandstones with unimodal grain size distributions, as well as facies with elevated percentages of silt and clay that are interpreted to be deposited as either cohesive debrites or as slurry deposits of transitional/co-genetic flows with fine-tail skewed grain size distribution. In these facies, we interpret the increased presence of fines to result from the trapping of fines within the suspended sediment fall-out as flow turbulence became less efficient in segregating grainsizes in this unconfined part of the fan. As a contrast, samples in the updip channel location (Site 621) are characterized by clean turbidite sands that lack any fine-tail skewed grainsize distribution.

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013