ABSTRACT

Liberty Field is located within the active up-dip Tuscaloosa trend of southwestern Mississippi and east central Louisiana. The Lower Tuscaloosa is composed of siliciclastic sediments derived from the Ouachita Orogenic Belt. These sediments were deposited by rivers which aggraded alluvial/deltaic plains and prograded moderate-size deltas into a shallow, rising sea. Much of the remaining undiscovered reserves within this mature oil play is believed to be present in small-to-medium-sized stratigraphic traps similar in size and style of occurrence to Liberty Field (OOIP of 7 million barrels of oil (MMBO)).

Oil at Liberty Field is stratigraphically trapped within the upper two Lower Tuscaloosa sandstones denoted, in descending stratigraphic order, as the "A" and "B" sandstones. Deposited during a transgressive period, these fluvial channel sands mark the passage from continental deposits into the marine shales of the Middle Tuscaloosa. Seismic sand mapping in conjunction with wireline logs from 30 wells and data from 15 conventional cores characterize the vertical succession of channel styles.

The A and B reservoir sandstones are medium-to-fine-grained, sublithic sands. Porosities range from 20 to 30 percent whereas permeabilities range between 35 to 800 millidarcies (md). Depositionally, the B sandstone represents a multi-storied, composite point bar and channel deposit. This sand, encased in floodplain silts and shales, was deposited along a low gradient, high sinuosity meander belt having a northwest-southeast trend. Individual channels range from 30 to 35 feet thick, whereas the maximum composite meander belt thickness is 55 feet. In contrast, the overlying A sandstone represents a simple single-storied point bar and channel deposit. Encased in shales containing a restricted marine fauna, this sand was deposited along high sinuosity distributary channel similarly trending northwest-southeast. Maximum channel thickness is 28 feet which commonly is marine reworked in the upper few feet. Meander belt width for both sands varies from 2,500 to 5,000 feet. Gentle structural setting (regional dip of 1°), channel style and orientation all influence hydrocarbon entrapment.