Oil and gas seeps in deep marine sea floor cores as indicators of active petroleum systems in Indonesia

Abstract

Sea-floor sediments collected during an extensive geophysical and coring project in several deepwater basins of Indonesia have been analyzed in detail using a range of geochemical techniques. The results show clear evidence of oil and thermogenic gas seepage, including basins or sub-basins that are largely unexplored. Biogenic gas was also commonly detected, although its presence was readily distinguished from thermogenic gas components. In many cases, oil was found in sufficient quantity to yield a full suite of biological markers. The biomarkers in turn were used to derive information about the parent source rock and to correlate the seep with known subsurface accumulations of oil. The ten areas of Indonesia examined in this study are: SW Sunda, North Makassar, Bone Bay, Banggai Sula, Gorontalo, Cendrawasih, Misool, Seram, Kumawa-Keras and Jamdena (Figure 1). The probable types of source rock for oil seeps in these areas have been determined using biomarker and isotopic compositions. Characteristics for both Mesozoic and Tertiary source rocks, including carbonate and clastic lithofacies, were observed. In some instances, the seeps tied directly to crude oils that are being produced in the basin. In other cases, the seeps showed sufficient difference to known subsurface accumulations to indicate a source rock of a different type or age was active. The sea-floor seeps have provided an extraordinary opportunity to gain insights into areas of active hydrocarbon migration in undrilled or under-explored basins. In some areas, this represents the first evidence for the existence of an underlying petroleum system, which in the future, may provide targets for wildcat exploration drilling. * Black Gold Energy LLC ** TGS-NOPEC Geophysical Company INTRODUCTION Oftentimes the single most important question to be addressed in the exploration of frontier or underexplored basins is whether or not an active petroleum system exists. Petroleum companies are generally reluctant to invest significant amounts of funds on exploration drilling unless an effective source rock or working petroleum system can be demonstrated. This creates a dilemma amongst exploration teams, since in order to provide the necessary evidence, well data and subsurface samples are usually required. Oil seeps in the proximity of geologically defined targets have historically provided sufficient incentive for exploration drilling. Crude oil at the surface is generally indicative of a working petroleum system at depth. A large number of successful discoveries have been made in the vicinity of oil seeps, particularly during the early years of oil exploration. As a result, the search for onshore seeps has become an integral part of the prospecting method, and it is safe to say that most of the world’s onshore macro-seeps have been observed and evaluated for their exploration significance (for Indonesian perspective, see Macgregor, 1995). In the present study, we extend the application of macro-seep detection to ten offshore areas (Figure 1). Recent technological advances have allowed the sea-floor to be precisely sampled in locations of probable hydrocarbon seepage (Orange et al., 2008). Modern geochemical analyses of seafloor cores provide evidence for thermogenic hydrocarbon seepage, and information on the type and origin of the hydrocarbons. We have examined several frontier offshore areas of Indonesia in this manner, in an effort to identify active petroleum systems and thereby reduce the risk for future exploration. © IPA, 2011 33rd Annual Convention Proceedings, 2009