ABSTRACT

Faulting which is contemporaneous with sedimentation and its associated structural closure serves as trapping components for much of the oil reserve remaining to be discovered in the Gulf Coast Province. In a recent two-year period 25% of the total discoveries in Southeast Louisiana were comprised of fault closure-type fields, and this percentage is believed to be increasing yearly.

An analysis of five fault closure-type fields in Southeast Louisiana reveals that the fields vary as to type of structure and producing fault block but there is one major factor which all have in common: all exhibit faulting which was contemporaneous with deposition of sediments, either wholly or in part. The analysis indicates that three types of fault closure are geologically and economically prospective: faulted noses or outer flanks of known structures which display (1) closure in a downthrown block, reservoir sands against shale, (2) closure in an upthrown block, reservoir sands against shale, and (3) closure in an upthrown block, reservoir sands against sand.

It is concluded from this study of fields illustrating the prospective types of fault closure that: (1) hydrocarbon accumulation can exist without dip reversal of reservoir beds into the fault, (2) accumulation can occur in sands which are upthrown against massive sands, and (3) the reserve potential of the three types of prospective fault closure is apparently equal when structural aspects alone are considered; however, the economics governing fault closure-type fields, as in any other type of field, depend on many variables, such as size, stratigraphic condition, and investments.

The fault closure-type fields illustrated have been selected from Southeast Louisiana, but the types of prospective fault closure, and the principles involved in their genesis, may be found wherever contemporaneous normal faulting occurs in a predominantly sand and shale environment, such as that existing in the Gulf Coast Province.