The generation of magnetic anomalies by combustion metamorphism of sedimentary rock, and its significance to hydrocarbon exploration

Abstract

Low-temperature exothermic oxidation of fine pyrite in carbonaceous marine rocks can lead to spontaneous combustion. As a result, the organic material in the rocks is consumed as fuel, but unlike burnt coal seams and lignite beds, the host rock remains after combustion. Thermal metamorphism, and in some cases partial to complete melting of the rock, will result. Zones of combustion metamorphism are frequently associated with oil fields. These occurrences can be related to either surface outcrops of source rock or leakage from reservoirs. A study of the magnetic properties of a suite of combustion-metamorphosed specimens of the Monterey and Sisquoc Formations in southern California shows orders of magnitude increases in magnetic intensity and susceptibility for both thermally metamorphosed and fused samples. These changes reflect both the production of new magnetic minerals, derived mainly from oxidized iron sulfide, and their acquisition of thermal remanence. Ground magnetic surveys across combustion metamorphism zones in three southern California oil fields reveal anomalies of up to 2,000 gammas associated with the metamorphosed and melted rock. Large-scale aero-magnetic anomalies are also associated with extensive combustion metamorphism zones in the Marcelina Formation of Venezuela and the Ghareb and Taquiya Formations (the “Mottled Zone”) in Israel. The recognition of magnetic anomalies caused by combustion metamorphism in areas of oil production suggests that closely spaced low-altitude aeromagnetic surveying can be a useful petroleum-exploration tool.