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DOWNTHROWN TRAPS OF THE NW WITCH GROUND GRABEN, UK NORTH SEA
Author(s) -
Hindle A. D.
Publication year - 1989
Publication title -
journal of petroleum geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.725
H-Index - 42
eISSN - 1747-5457
pISSN - 0141-6421
DOI - 10.1111/j.1747-5457.1989.tb00240.x
Subject(s) - graben , geology , fault (geology) , petroleum reservoir , petrology , seismology , paleontology , structural basin
The structures drilled to‐date in the NW Witch Ground Graben area have been assigned to two broad categories: (i) Upthrown traps with four‐way dip closure or tilted fault blocks, generally located on the franks of the graben: and (ii) Downthrown traps with closure against a major fault, located downthrown on the main graben border fault systems. Many of the downthrown traps are in fact combination traps, having upthrown fault‐seal and/or stratigraphic elements. A study of trap settings in the NW Witch Ground Graben has led to the following conclusions: 1. The success rate for encountering hydrocarbons in upthrown traps and downthrown traps is very similar. 2. The common belief that the footwall seal is the most critical element of a downthrown trap is questionable. 3. Modern seismic data reveals that a high proponion of unsuccessful exploratory wells were not located on a closed structure. The proportion is similar for upthrown and downthrown trap targets alike. 4. There is a lesser chance of the reservoir being absent, or there being no migration route to the trap, in downthrown structures, compared with upthrown ones. 5. Carboniferous sediments can be a footwall seal for a downthrown trap in one location, and a reservoir for hydrocarbons in another. 6. The structural evolution of the NW Witch Ground Graben, within a framework of three intersecting fault trends, has been ideal for the development of downthrown traps. The distribution of a hydrocarbon source rock, the timing of hydrocarbon migration into these structures, and the suitable distribution of sealing strata adjacent to the border fault of the structure, have also been critical.