Accumulation of oil and gas in an area depends on amount and type of organic matter, adequate temperatures for generation, suitable trapping configuration, and correct timing of events. All these factors can vary considerably across active margins of both island arc and continental (Andean) type.

Forearc areas are characterized by low geothermal gradient owing to subduction, poor reservoirs derived from volcaniclastics, and relatively low organic carbon content. Although tectonic complexity may offer a wide variety of trapping configurations, overall petroleum potential is low. Gas is present in some commercial (and many noncommercial) accumulations and is in part biogenic. What little oil is present is usually paraffinic with a low sulfur content (^approx 0.1%) and an API gravity in the range 30°-35°. These facts suggest a major role for land-derived organic matter, an idea supported by the available geochemical data.

Back arc areas are characterized by higher geothermal gradients and

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larger basins. These may show evidence of extension with the development of organic-rich lakes or marine embayments in the downthrown area. Here source rocks are shales with above average organic carbon content. They have generated high pour-point waxy crude oils with low sulfur content and API gravities around 35°. Again, these characteristics suggest an important role for land-derived organic materials and this is supported by geochemical data for biomarker distributions, pristane/phytane ratios, etc. The simpler structural setting in back arc basins favors the development of larger fields, and several giants occur in this setting, for example, Minas in Sumatra.

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