A Geochemical Model For Characterization Of Hydrocarbon Gas Sources In Marine Sediments

Abstract

ABSTRACT A geochemical model has been developed to distinguish biogenic from petroleum-related gas in marine sediments. Analyses of seep and sediment gases from the Gulf of Mexico indicate that the characteristic composition of pathogenic gas can be altered during migration through sediments and by mixing with biogenic gas. Measurements of carbon isotopic ratios of methane and hydrocarbon ratios generally provide a definitive interpretation of hydrocarbon gas sources. It is suggested that prospecting for reservoir hydrocarbons include coring on or near observed geologic features to first establish the source of existing hydrocarbon gas in a region. INTRODUCTION An extensive review of major petroleum provinces by Link (1952) indicates that most important oil-producing regions have been associated with oil and gas seepage. Thus, the detection of seepage has provided the initial clues to the discovery of most of the world's major oil fields. Over the past twenty years, various geochemical methods of prospecting for oil and gas seepage from reservoirs have been introduced, including measurements of adsorbed and occluded hydrocarbons from soil (Horvitz, 1954) or marine sediments (Gerard and Feugere, 1968), "sniffing" for dissolved light hydrocarbons in seawater (Brooks and Sackets, 1973; Sigalove and Pearlman, 1975), and analysis of light hydrocarbons dissolved in interstitial water (Carlisle et at., 1975). These exploration techniques are made theoretically feasible because hydrocarbons from petroleum reservoirs migrate upward through faults, fractures, bedding planes, etc., and exude from surface sediments into the overlying waters. Thus, the detection and characterization of marine seepage may reveal the location of offshore oil and gas deposits. In theory, petroleum-related (petrogenic) gas can be distinguished from microbial-produced (biogenic) gas by the presence of significant quantities of ethane-and-higher hydrocarbons. The analyses reported here suggest that the hydrocarbon composition of pathogenic gas can be altered to resemble biogenic gas during migration through the sediments, thus confusing interpretation. Moreover, mixtures of gases from the two sources can retain the character of petroleum-related gas in hydrocarbon composition while changing isotopically. Since the measurement of one parameter such as hydrocarbon composition can be misleading, an examination of two or more parameters can aid in characterization of gas origin. Analyses of isotopic composition of methane from seepage and sediment gas in conjunction with molecular composition can provide more definitive interpretation. GENERAL PRINCIPLES Two types of hydrocarbon gas present in Gulf of Mexico sediments have been described by Bernard et at. (1976). Biogenic gas consists almost exclusively of methane, carbon dioxide, and hydrogen sulfide, having Cl/(C2+C3) ratios rater than 1000 and carbon isotope ratios (? 13C values) more negative than -60%.Petroleum-related hydrocarbon gases generally have Cl/(C2+C3) ratios smaller than 50 and isotopic ratios greater than -50 000 Since these two parameters should be diagnostic of the origin of the gas in seep or sediment samples, a simple geochemical model for the characterization of hydrocarbon gas sources can be constructed from the field of Cl/(C2+C3) ratios and ? 13C values found in marine natural gases.