ABSTRACT

Detailed analysis of sedimentary thicknesses at Grand Lake Field has revealed that hydrocarbon accumulation was controlled by faulting which was related to diapiric uplift of shale.

Grand Lake Field is located in the northeastern corner of Cameron Parish, Louisiana. This area contains about 12,000 feet of Miocene and younger fluvio-deltaic sediments. Structurally, the field is a northwest trending anticline. Diapiric shale in the western part of the field may be salt-related, although to date no salt has been penetrated. A major down-to-the-south regional growth fault crosses the top of the structure, striking roughly northwest. Several down-to-the-north faults are antithetic to this master fault. Second and third generation antithetic faults also are present in the field.

Diapiric uplift in Grand Lake Field was initiated in the Early Miocene by an influx of relatively heavy deltaic sands onto undercompacted shales. The master fault in the field formed almost immediately after the onset of uplift, and movement continued essentially uninterrupted until the Plio-Pleistocene.

The type of structural assemblage which is found at Grand Lake Field is common in the Gulf Coast of western Louisiana and Texas. These assemblages developed as a consequence of a mechanism known as gravity sliding. The master fault formed on the basinward flank of a vertically uplifting shale structure as a result of gravitational forces. The complex arrangement of antithetic faults in the field is due to stress adjustment, caused by movement of the master fault. Production in Grand Lake Field is from a number of sandstones of Early and Middle Miocene age. The distribution of oil and gas within Grand Lake Field appears to be related to the sealing properties and timing of motion of the first-order antithetic faults at the time of migration of hydrocarbons into the field.