Delineation of a Massive Seafloor Hydrocarbon Seep, Overpressured Aquifer Sands, and Shallow Gas Reservoirs, Louisiana Continental Slope

Abstract

Abstract Seafloor and sub-seafloor interpretive maps derived from broad-band 3D short-offset seismic augmented with seabed piston core data were utilized to assess potential hazards and constraints to both mobile drilling and field development at a Louisiana Continental Slope location. Numerous isolated and coalesced mud volcanoes with associated mudflow deposits are observed within an elongate seafloor depression bound on three sides by complex curvilinear faults and fractures. Rim synclines are associated with the two largest mud volcanoes. Total signal attenuation, velocity pull-down, and absence of a top-of-salt reflections characterize the 3D geophysical response below these features. Geotechnical analysis of several piston cores acquired at the mud volcanoes and flanking mudflows reveal oil staining, gas-fracturing, carbonate inclusions, and minor gas-hydrate within a silty-clay matrix. Geochemical analyses revealed anomalous concentrations of live-oil, biodegraded oil, and C1 through C4 gases. Subsurface observations include at least two slope channel/fan sequences within the shallow sedimentary section associated with a complexly faulted half-graben. Sand-prone sediments are interpreted from seismic facies characterization that include incised basal unconformities, low-impedance moderate to high amplitude reflections in downlap, mounded, chaotic, and channel/levee orientations. Low-impedance amplitude anomalies occur in structural and stratigraphic traps within the footwall and hanging-wall of the graben. A thick clay-rich channel and debris flow sequence overlies the slope channel/fan sands. These observations are interpreted to represent an established and long-lived hydrocarbon seep trend whose primary migration path is a series of deep-seated faults forming a half-graben in the near surface. Active surface mudflows and "fresh" oil recharge at the seabed demonstrate ongoing growth of the mud volcanoes. The study area has been a depositional fairway for submarine channel/fan sands that have been brought into juxtaposition with a massive hydrocarbon seep. Inferred rapid sedimentation rates and clayrich overburden, potentially acting as a seal, suggest that abnormal pore-pressure could be associated with the underlying sands. Subsequent drilling confirmed the presence of overpressure. In updip and stratigraphic orientations, these same sands are charged with gas. Mudflows represent a potential foundation hazard to seafloor assemblies while overpressured water and gas sands represent a potential wellcontrol hazard. Introduction This paper discusses the analysis of 3D seismic and seabed piston core data utilized for a mobile drilling rig site investigation at Garden Banks Blocks 741 and 785 on the Gulf of Mexico Continental Slope. The study area covers approximately 64 square miles that includes all or portions of Garden Banks Blocks 696,697,698, 740, 741, 742, 784, 785, 786, 828, 829, and 830. Data Resources. Various forms of 3D seismic data are increasingly being utilized worldwide for geohazards analysis4,5,7,8,14,16 In this case, a resolute 3D Short-Offset seismic volume was reprocessed from a 3D speculative exploration data set6 The original exploration data was conventionally processed (full-fold BIN stacking) at a 4 millisecond sample rate, and output with a CDP/Inline spacing of 41 by 131 feet. Maximum apparent frequencies in the shallow section were 65Hz. The 3D Short-Offset volume was processed from the field tapes at a 2-millisecond sample rate. Only near-incident traces stacked at their true CDP locations (non-binning technique) were utilized to produce 3-fold stack.