SCHUMACHER, DIETMAR, Pennzoil Exploration and Production Company, Houston, TX
The presence of magnetic anomalies over oil and gas fields has been noted for several decades, but it is only in recent years that the phenomenon has been critically examined. Studies of geologically and geographically diverse regions show that (1) authigenic magnetic minerals occur in near-surface sediments over many petroleum accumulations, (2) this hydrocarbon-induced mineralization is detectable in high resolution, broad bandwidth magnetic data acquired at low altitude and with closely-spaced flight lines, (3) the magnetic susceptibility analysis of drill cuttings and near-surface sediments confirms the existence of the aeromagnetic anomalies, and (4) sediments with anomalous magnetic susceptibility frequently contain ferromagnetic minerals such as greigite, maghemite, magnetite, nd pyrrhotite.
The formation of magnetic minerals under conditions of hydrocarbon seepage is a very complex process and still poorly understood. The specific minerals formed depends on many factors, and the stability of these authigenic magnetic minerals is also variable. For example, non-magnetic species such as pyrite and siderite can be oxidized to more magnetic minerals, whereas some magnetic species can under highly reducing conditions be altered to pyrite or siderite.
The association between hydrocarbon seepage and the formation of authigenic magnetic minerals in the near-surface has important applications in hydrocarbon exploration. Aeromagnetic delineation of potential hydrocarbon accumulations is a rapid, cost-effective method for surveying and evaluating large areas. Although the discovery of shallow magnetic anomalies does not guarantee the discovery of hydrocarbon accumulations, it does identify areas requiring more detailed evaluation. Proper integration of near-surface magnetic data with geologic and seismic data can improve exploration success and reduce development costs.
AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.