Organic matter provenance
Organic matter in sedimentary basins, usually marine and either of Recent or geologically old origin, is derived from the syngenetic residues of posthumus biogenic debris (Simoneit, 1982a; Tissot and Welte, 1984; Hunt, 1996). This material is composed of both autochthonous detritus and allochthonous residues derived from continental sources (Simoneit, 1982a). Aquatic sediments receive allochthonous organic detritus primarily by river wash-in and eolian fallout particles, with ice-rafting and sediment recycling as minor contributing processes (Simoneit, 1975, 1978). Organic matter that accumulates in contemporary sediments represents the residues from primary biological carbon fixation and its degradation (remineralization; Table 1). The nature of this immature organic material is described below, followed by a general description of more mature organic matter formed as a result of maturation in subsiding basins.
Nature of immature organic matter in sedimentary basins
Gas: Interstitial gas in recent sedimentary environments consists primarily of methane, carbon dioxide, and sometimes hydrogen sulfide (Claypool and Kaplan, 1974; Claypool and Kvenvolden, 1983). The biogenic hydrocarbon gases usually have CH4/(C2H6 + C3H8) ratios greater than 1,000, while those of a thermogenic origin have ratios less than 50 (Bernard et al., 1976). For example, the C1/(C2 + C3) ratios for shallow sediment gases from Guaymas basin, Gulf of California range from 41 to 150 and thus indicate a mixed origin of biogenic (CH4) and thermogenic (C1-C8) hydrocarbons (Simoneit et al., 1979). The depth range where biogenic gas can be found is variable but generally shallow (∼100 m) and depends on microbial production and environmental conditions in the sediments.