Probability of Economic Success for Wells Drilled Below 18,000 Ft in Southern Louisiana

W. G. Leach, W. H. Fertl

Many geologic structures in South Louisiana are thought to contain major potential reserves at depths in excess of 18,000 ft. A detailed study was performed to statistically evaluate and establish discernible trends as to the effects of formation pressure, temperature, and depth on the commerciality of hydrocarbon reservoirs in southern Louisiana.

At the time of this study, 515 southern Louisiana wells had been drilled below 18,000 ft to Miocene and Oligocene target zones. Seventy-six wells, located in eight coastal parishes, were completed below 18,000 ft. A total of 146 wells were drilled as dry holes in conjunction with these 76 completions either on the same geological structure or in the same field. Of these 146 wells, 65 of them were dry holes and 81 had dryhole footage below 18,000 ft. These 222 wells were selected as the basis for the economic evaluation.

Abnormally high formation pressures, generally prevalent in southern Louisiana below 18,000 ft, were found to significantly affect recovery of these deep hydrocarbon reserves. The economic criteria were correlated to the well-log-derived shale resistivity ratio (SRR). The SRR is the ratio of the predicted "normal" shale resistivity divided by the observed "actual" shale resistivity.

The SRR value for wells below 18,000 ft can be used as an input parameter for economic managerial decisions on whether to stop or continue drilling. In the present 222-well study, the change of pay-out for a well is reduced by 80% once the SRR exceeds 3.5. Concurrently, the chance of finding greater than one billion ft³ of gas-equivalent is also decreased by 80% once 3.5 SRR is reached.

No well has ever had an economical offset well within the same reservoir (or fault block) where the SRR exceeded 3.5. Furthermore, no well exhibiting an SRR less than 3.5 has ever been perforated below 20,700 ft. With the exception of the two depositional centers of the upper Miocene in Terrebonne Parish and the Mary "A" sand series trend, primarily in St. Mary Parish, the lack of economic success below 18,000 ft is caused by lenticular, shaly and silty sands located in the middle to outer neritic depositional environment.

AAPG Search and Discovery Article #91030©1988 AAPG Annual Convention, Houston, Texas, 20-23 March 1988.