From Bureau of Economic Geology, The
University of Texas at Austin (www.beg.utexas.edu).
Bureau Seminar, November 30, 2007
New Interpretations of Reservoir Architecture in East Texas Field: Sequence Stratigraphic and Depositional Perspectives
and Bureau of Economic Geology, Jackson School of Geosciences
The East Texas Field, located on the west margin of the Sabine Uplift, is the largest oil field in the lower 48 states and produces from a classic stratigraphic trap of Woodbine Group sandstones. Productive units occur within the lower Woodbine, the primary coarse-siliciclastic interval of the Upper Cretaceous in the East Texas Basin. Correlation of fourth-order sequences from the thickest Woodbine in the basin center to the uplift indicates that reservoirs compose the basal three fourth-order sequences of a succession that contains a maximum of 14 such sequences in the basin axis.
Analysis of >1,500 ft of 30 whole cores and wireline logs from approximately 500 wells indicates that the sandstone-body architecture is more complex than inferred from previous studies. Moreover, these data, in concert with sequence-stratigraphic analysis, show that the depositional settings of reservoir facies vary considerably from those of earlier investigations.
Extreme sandbody heterogeneity is controlled by the fluvial-dominated deltaic depositional architecture. This highstand section is truncated in the northern and western part of the field by a thick (100 to 150 ft), conglomeratic lowstand incised-valley-fill succession, which records as much as 150 to 200 ft of drop in relative sea level. Previous studies of the Woodbine inferred well-connected, laterally continuous sheet sandstones in a wave-dominated deltaic and barrier-strandplain setting. This wave-dominated deltaic model is inappropriate, and a full understanding of reservoir compartmentalization, fluid flow, and unswept mobile oil in East Texas field should consider the fluvial-dominated deltaic and lowstand valley-fill sandbody architecture.