Lockhart Crossing Field: A Serendipitous Discovery

Dan J. Hartmann
DJH Energy Consulting, Fredericksburg, TX

Lockhart Crossing Field is an oil accumulation of Wilcox age sandstone that had been penetrated by a few deeper wells, but not completed. The wells contained a thin bar facies with low resistivity at the base. The consensus was that this was a marginal to uneconomic zone, and not worth the cost of completion.

A later exploration well drilling to a deeper objective, became a SERENDIPITY discovery, encountering a thick channel facies across the bar, charged with oil.

Consequently, this field had a long primary production history, followed by a partially successful waterflood. How could this accumulation have been recognized?

Evaluating data from the original wells using an assessment procedure would have yielded:

1. A valid Rw number from the SP log, to calculate Sw.
2. Detailed cuttings descriptions and photos would have revealed the geometry of the clay cemented pores.
3. Sw calculated using Archie would have shown values approaching only 50% in the "wet" portion of the bar.
4. Since the three initial wells were at different structural elevations, a plot of Sw to subsea depth for those wells would have revealed that the highest Sw occurs at the highest elevation in the bar.
5. Integration of the structural and stratigraphic model from the 2-D seismic might have set up the Wilcox as a primary exploration objective, before serendipity saved the day.