ABSTRACT

An interpretation of depositional environments and a general depositional history of the subsurface Frio in South Texas is presented.

In the South Texas subsurface Frio the reservoir rocks appear to be very closely related to deltaic environments. In the updip Frio the hydrocarbons are found mainly in alternating deposits of:

1. Non-marine environments (including deltaic plain, distributary, alluvial, and lacustrine).
2. Coastal environments (including brackish water bays, beach, inter-distributary, and delta-front).

In the downdip Frio the hydrocarbons are found mainly in alternating deposits of:

1. Inner-neritic environments (including open ocean beach, some brackish water, and delta-front).
2. Middle-neritic environments (mainly pro-delta).

The subsurface Frio is believed to be the equivalent of the Bucatunna and upper and lower Chickasawhay Formations that are exposed in Mississippi and Alabama.

Evidence is presented to indicate that source beds and reservoir rocks may be one and the same, or at least a very close relationship exists, and that there has not been any appreciable long-distance migration of hydrocarbons.

The importance of the documentation of the depositional environment of mineral bearing sediments is stressed.