Distribution of Light Hydrocarbon in Seafloor Sediments: Correlations Between Geochemistry, Seismic Structure, and Possible Reservoired OiI and Gas
Authors: Carlisle Charles T. , Bayliss G.S. , VanDelinder D.G. 
Published: October 2008

INTRODUCTION

Geochemical surface prospecting has historically aroused the interest of the petroleum industry as potentially one of the most direct oil finding methods. Over the past forty years, almost every conceivable surface prospecting technique has been evaluated by research and exploration groups of almost every major oil company. In spite of all the findings, pro and con, geochemical surface prospecting remains highly controversial as being a direct method of finding subsurface oil accumulations.

Much of this controversy can be directly attributed to the analytical techniques, or to the methods of interpreting the geochemical data. In most cases, such criticism is well founded. Many methods used in surface prospecting are indeed complex, and do require highly sophisticated experimental technology. Of the geochemical surface prospecting methods documented in the literature, the most Widely used techniques involve measurement of absorbed C1-C5 light hydrocarbon in terrestrial and marine samples (Horvitz, 1954); "gas-sniffing" for dissolved CrC5 light hydrocarbon in sea waters (Dunlap, 1960; Sigalove, 1972); and the extraction and spectrofluorometric measurement of heavy (C15+) hydrocarbons present in muds and soils (Briecker, 1962). Similarly, in many cases, very detailed statistical treatment of the data is necessary before the geochemical anomalies of interest may be fully defined.

Irrespective of the many criticisms levied against the application of these techniques, however, all have one thing in common. That is, independent of whichever technique is used, the data clearly shows that anomalously high values which are significantly above background are found in many areas. Furthermore, these anomalies are not only mappable but closely related to geologically defined features such as faults, mud lumps, salt domes, anticlines, etc.

Another surprising aspect of geochemical surface prospecting is that is not more universally accepted as an exploration tool, since scientifically, the entire concept is based on the simple premise that lk1uid and gaseous hydrocarbons resenvoired in subsurface sediments are leading-a fact which is historically well proven. The early recognition of onshore oil and gas seepage being an indication of subsurface accumulations seemed obvious to the early explorationist, and as a result, many of the world's great oil fields were initially found by seepage drilling (Link, 1954). The basic concept of micro-seeps, which involve hydrocarbon leakage at levels well below the range of visual detection, must be considered valid in order for surface prospecting to be acceptable. Similarly, in accepting the micro-seep concept, one also accepts that the location of such micro-seeps should be on or near geologic features (such as unconformities, bedding planes, faults, etc.) which act as conduits by which deep reservoired hydrocarbons vent to the surface.

NEW APPROACH

After reviewing all available published procedures and basic concepts governing geochemical surface prospecting, a new, hitherto unreported, method has been developed, which has direct application to finding oil and gas accumulations offshore. The technique primarily involves the analysis of "free" C1 (methane), C2 (ehtane), C3 (propane), 1C4 (isobutane), nC4 (normal butane) and the C5-C7(pentane through toluene) hydrocarbon present in surface sediment samples collected in offshore areas of exploration interest.