Analysis and Interpretation of High-Resolution Geophysical Data: Offshore Sarawak, South China Sea
Authors: Zhang Zijian , Digby Adrian John , Gharib Jim J. , Chong Tai Foi 
Published: April 2012

Pockmarks and related subsurface features are mapped and interpreted inoffshore Sarawak, South China Sea. Data was collected using sidescan sonar, sub-bottom profiler, 2D high-resolution seismic and core samples. Pockmarkdistributions are interpreted and defined on the basis of these high-resolutiongeophysical data in three zones: near shore with ~20–30 m water depth, offshoreshallow water with 80–90 m water depth, and offshore deep water with ~150–500 mwater depth. We propose that pockmark formation is controlled by overpressuredlayers, faults and fractures, shallow gas accumulation in shallow sand layers, and sediment deposition rates. Of the three zones the offshore deep water zoneis characterized by relatively higher overpressure, distinct seafloor faults, and a low sediment deposition rate; it has the densest pockmark concentrationof the three zones. Although shallow gas and shallow sand layers areinterpreted in the near shore zone, it is characterized by lower overpressure, an absence of well defined seafloor faulting, and relative higher sedimentdeposition rate. No obvious pockmarks are visible in the near shore zone. Understanding pockmark distribution and formation mechanisms will help inassessing potential geohazards and defining strategies for efficientidentification of oil and gas seeps in seep-hunting project.

1. Introduction

Fluid seeps are natural occurrences in which fluid migrates from the subsurfaceto the seafloor along permeability pathways created by tectonic activity andnatural hydraulic fracturing. Typically, the fluid seeps are divided into threecategories:oil- and gas-dominated systems that provide true directhydrocarbon indicators, which aid the oil and gas industry's investigation anddetection of hydrocarbon deposits;water-dominated systems that can alsocontain dissolved hydrocarbons, carbon dioxide, nitrogen and hydrogen sulfide;andbrine-dominated systems associated with subsurface salt (Moore, 1999). Seep-hunting techniques have been used by deepwater oil and water operatorsbecause the relatively inexpensive techniques can reduce significantly the riskof investing in frontier basins (McConnell et al., 2008).

Pockmarks and shallow thermogenic gas accumulations within sediments are twocommon seafloor and near-surface expressions of fluid seeps. Pockmarks areconcave, crater-shaped seafloor depressions that are generally related to fluidand/or gas escaping to the seafloor (King and MacLean, 1970). They are found inactive and passive continental margins from shallow water to deep water allover the world (Judd and Hovland, 2007). Shallow gas in marine sediments isdivided into biogenic gas and thermogenic gas (Rice and Claypool, 1981). Biogenic gas is mostly composed of methane whereas thermogenic gas is composedof methane with varying amounts of higher hydrocarbon gases. Biogenic gas isoriginally produced in organic-rich sediments within a few hundred meters belowthe seafloor. Thermogenic gas forms within reservoir rocks at high temperaturesand high pressures at depths generally greater than 1000 m. The thermogenic gasmay migrate towards the surface and accumulate in shallow sediments.