Pockmarks are natural bottom features, unrelated to production activities. Physical evidence for the origin of pockmarks from three sites in the northwestern Arabian Gulf is examined. Three related features are also discussed. Two sources of gases are suggested by the physical evidence.
Since King and MacLean (1970) first applied the term "pockmark" to crater-like depressions found on the Scotian Shelf, similar features have been described in the North Sea (Hovland, 1981a, b; McQuillin and Fannin, 1979; Green et al., 1985), the East China Sea (Milliman et al., 1985) and off Alaska (Sandstorm et al., 1983). Pockmarks seem to be associated with hydrocarbon-bearing areas, although a legitimate question to be asked is whether this is a function of geology or survey coverage. Pockmarks are of interest to oil companies as they may pose a threat to seabed installations.
It is now recognized that most pockmarks result from the erosion of the sea floor by venting fluids, which in most cases is presumed to be gas. The gas has two possible origins: petrogenic (thermogenic) and biogenic. In Norton Sound (Alaska), for example, gases contained in the sediments of a pockmarked bottom have been shown to be biogenic (Sandstorm et al., 1983). Gas-charged sediments need not lead to the formation of pockmarks. Gas associated with peat deposits in Long Island Sound (New York/Connecticut) has not led to the formation of any recognizable pockmarks (Bohlen, 1985). Little is known of the role of bottom dwellers and feeders in either the creation or maintenance of these depressions. A key question to be answered then, is why pockmarks form where they do.
Aramco conducts two types of surveys : site-specific and regional. In both cases, bathymetric side-scan sonar and Uniboom data are collected. Regionals, or field-wide surveys have a 100 m line spacing. Site-specifics, which are one kilometer squares centred on some point of interest, have a 50 m spacing for Uniboom and sied-scan lines. The operational areas shown in Fig 1, which encompass 1300 km2, are regions in which surveys have been conducted. Except in deeper waters, where seismic penetration can be in excess of 40 m, shallow seismic data is to depths of less than 30 m.
A large number of engineering soil borings have been drilled within the Aramco concession area. Generally, the borings are about 40 m deep. Surface sediments range from very soft, lime mud to beachrock (locally called "faroush") and coral. Subsurface sediments are typically carbonate sands and clays. Cemented sands, mudstone and limestone are quite common, occurring in thicknesses ranging from thin partings to thick bed sequences. Evaporates are present in some of the borings from deeper waters in the form of gypsum and anhydrite. The presence of H2S is often noted in bore logs. It is usually associated with surface and near-surface deposits. At some locations, dulfur has been observed to be a cementing agent.
Fig. 1 The Arabian Gulf operational areas are zones in which Aramco has undertaken surveys (available in full paper)
