Natural catalytic activity in a marine shale for generating natural gas

Abstract

Many organic-rich rocks are major sources of oil and gas in sedimentary basins presumably through high-temperature thermal cracking. This view was brought into question with recent reports of marine shales generating catalytic gas in the laboratory at 50 ° C, 300 ° C below thermal-cracking temperatures. Gas forms under natural conditions without artificial stimulation. Compositions of methane, ethane and propane are near thermodynamic equilibrium (2C 2 H 6 =CH 4 +C 3 H 8 ) mirroring those in natural deposits. It is significant because thermal cracking can neither generate hydrocarbons at equilibrium nor can it bring them to equilibrium over geological time. Thus, catalysis must be the source of equilibrium in natural gas habitats and in marine shales. There is experimental evidence for metathesis ( ) as the catalytic path to equilibrium. However, it is without example in contemporary catalysis, and therefore, calls for extraordinary empirical support. Here, we report independent and unequivocal evidence of natural catalytic activity in a marine shale linking metathesis and thermodynamic equilibrium. A Cretaceous Mowry shale catalysed the dimerization of propylene (C 3 H 6 ) to methyl cyclopentane (MCP, C 6 H 12 ) and n -hexane ( n -C 6 , C 6 H 14 ) at 50 ° C in greater than 99 per cent selectivity. Propylene increased the rate of n -C 6 generation by a factor of 100 with 100 per cent selectivity to the straight-chain hexane ( n -C 6 ). Propylene also suppressed the generation of all hydrocarbons except cyclopentane, MCP and n -C 6 . The ratio MCP/ n -C 6 , which swung chaotically between 1 and 25 before propylene addition, was rendered invariant with propylene addition ( R 2 =0.99; MCP/ n -C 6 =1.20±0.034 s.d.). These uniquely catalytic reactions confirm natural catalytic activity in this shale. It appears to be ‘palaeoactivity’ possibly conceived in early diagenesis and sustained over geological time.