The "wedge belts of permeability" that A. I. Levorsen emphasized as being prime targets for petroleum exploration have not become the objects of systematic exploration efforts, possibly because many geologists do not readily understand how petroleum can be found in an angular-unconformity trap that was subaerially eroded. The thesis of this paper is that modern geochemical research about kerogen, mostly isolated from supposed "source-rock" shales, can be applied equally to the kerogen found within permeable formations. If this concept is true, then many of the geologic enigmas associated with angular-unconformity traps can be explained.
Immature kerogens within sands possibly can be subaerially exposed and/or flushed through with groundwater without being significantly altered. Because it does not migrate, solid kerogen within the permeable potential reservoir formation would not be lost updip when the future reservoir-to-be was exposed at the land surface. Moreover, any immature kerogen in the pre-unconformity permeable strata that survived weathering would be available for delayed conversion to petroleum at depth after geothermal heating either during the cycle of subsidence when the covering strata were deposited or later. In the origin of such petroleum, the only migration required is updip within the permeable formation to the sealed unconformity trap—Levorsen's wedge edge.
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The Deliberate Search for the Subtle Trap
The papers included in this volume reflect both the geological and geophysical rationale used in searching for the subtle trap. The scope of the papers ranges from the general appraisal papers of new concepts and methods, to those relating to specific fields in the United States, Nigeria, China, Australia, Canada, Oman, offshore Spain, and the North Sea. The 21 chapters are sourced from a 1981 AAPG Annual Meeting session, dedicated to providing explorationists with the information needed to search for and discover the stratigraphic, paleogeomorphic, and unconformity-oriented subtle-trap accumulations of oil and gas.