Petrophysical and sequence stratigraphic analyses of the Nacogdoches Oil Field in Nacogdoches County, Texas

This is a study of the Nacogdoches Oil Field in Nacogdoches County, East Texas. The study attempts to determine the nature of the hydrocarbon trapping mechanism for the field, the depositional environment of the Eocene strata from which it produces and the sequence stratigraphy of those units. For this research, stratigraphic and structural cross-sections of Eocene units in the oil field were constructed using 32 well logs.;The major structural features near the Nacogdoches Oil Field include the Sabine Uplift to the north and east, the East Texas Basin to the northwest, the Angelina Flexure to the south and Mt. Enterprise-Elkhart Fault System about 30 miles (48 km) to the north. Overall, the geology of the Nacogdoches Oil Field is composed of sub-horizontal sedimentary rocks of the Cenozoic Eocene Claiborne Group that generally dip at 1--2° to the south or southeast.;The main units of interest in the Nacogdoches Oil Field are a thick succession of Eocene non-marine and shallow marine units. Major stratigraphic units observed on the electric logs include, from the oldest to youngest, the Eocene Reklaw, Queen City, Weches, Sparta and Stone City/Cook Mountain Formations. They were deposited during episodic transgressions and regressions of the Claiborne shoreline and range in thicknesses from tens to hundreds of feet or (tens of meters). Sedimentary deposits of the Nacogdoches Oil Field are located in the eastern transitional zone between the mid-Cenozoic Rockdale delta system to the west and the Holy Spring delta system to the east. These deposits are mainly deltaic so they do not show pronounced vertical succession of facies. Instead, they show a significant lateral succession of facies that tends to persist vertically. This starts with the continental Carrizo unit which was deposited in a delta-barrier island setting by streams or longshore currents that deposited their sediment on a level coastal plain and developed a broad alluvial apron throughout the coastal length. Overlaying the Carrizo Sandstones are littoral and marsh/swamp deposits of the Reklaw Formation through fluvial-deltaic deposits of the Queen City Formation. Claiborne Group deposition continued with the emplacement of marine and littoral shelf deposits of the Weches Formation followed by littoral and beach deposits of the Sparta Sandstone through marine and littoral Stone City/Cook Mountain Formation deposits, and finally to sandy continental Yegua Formation deposits. Accommodation space and sequence development in this setting were controlled mainly by gradual subsidence of the shales and variations in sea level.;The Queen City Formation is the reservoir unit for the shallow Nacogdoches Oil Field. Together with other Claiborne units, the Queen City in the study area was deformed into an anticlinal or dome structure, which is revealed in both the structural maps and cross-sections made for this study. The lower Tyus Member of the Weches Formation probably serves as the caprock that prevents hydrocarbons trapped in the Queen City Formation from migrating to the surface. The anticline in the Nacogdoches Oil Field may be associated with movement of under-compacted shale units within the lower Queen City and Reklaw Formations. Thus the trapping mechanism for the oil field may be structural, stratigraphic, or a combination of both.