SOURCE ROCKS AND THE ORIGIN OF NATURAL GAS IN THE FAR EAST

Abstract

Most source rocks of the Far East (Figs. 2 and 6) have a strong humic component, and, therefore, a large gas‐generating capacity. This is an obvious reason why the Far East as an entity is largely a gas province. In particular, the following statements can be made: (i) Mixed or humic type source rocks in combination with temperature gradients exceeding 3.5°C and minimal burial depths of 3,000m generate predominantly gas (e. g. Gippsland basin, SE Australia; Kapuni gas field, New Zealand). (ii) Mixed or humic type source rocks in combination with average or slightly below average temperature gradients and burial depths exceeding 3,200‐4,000m generate predominantly gas and condensate (e. g. Dampier sub‐basin, Western Australia; Arun gas field, North Sumatra). (iii) Mixed and/or humic type source rocks generate gas in the vicinity of magmatic intrusive or extrusive bodies provided magmatic activity post‐dates source rock deposition. (iv) Most source rocks of the Far East are Paleocene to Miocene age, and source rock maturity and post‐maturity was in many cases reached in Neogene to Recent times. (v) Mesozoic source rocks of partly humic or mixed type are known from Afghanistan and Australia, Paleozoic source rocks from Australia. (vi) Retention of the gas phase may have been inadequate in areas of strong Neogene folding (e. g. Sumatra, East Kalimantan and Burma Tertiary basins). (vii) At the present state of knowledge the question as to which part of natural gas is thermally degraded oil and which part stems from a gas source rock, cannot be satisfactorily solved.