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

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 C, through C 4 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 down lap, 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. 37These 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.

Hydrocarbon leakage interpreted on seismic data

“…An amplitude anomaly is defined as a local increase or decrease of seismic reflection amplitude. Definitions of the terms can be found in Hovland and Judd (1988), Sheriff (1991) and Corthay (1998).…”

Diapiric Structures in the Tinto River Estuary (SW Spain) Caused by Artificial Load of an Industrial Stockpile

“…In our case, the structures described have been interpreted for the first time as the result of the vertical migration of fluids and fluidized sediments due to overpressure from a pile of industrial waste. However, these structures present morphologies similar to those observed on some continental platforms and are attributed to gas and water extrusions (e.g., [20][21][22][23][24][25][26]). Milkov [27] was the first to consider the extrusion of fluids as a generic phe- The piling of 35 m of industrial waste on the marsh surface in just 40 years has doubled the load supported by the sediments of the lower units of the estuarine fill [7].…”

An Overview of Mud Volcanoes Associated to Gas Hydrate System

“…The association of gas hydrates with submarine mud volcanoes was first noted by Reference [145] and has since observed in the Caspian [42], Black [27,[146][147][148], Mediterranean [57,148], Norwegian seas [48,58], offshore Barbados [159], offshore Nigeria [160] and in the Gulf of Mexico [161,162]. There are many common features of gas hydrates associated with mud volcanoes.…”