Abstract

It has been well documented that most oil and gas accumulations leak hydrocarbons, that this leakage (or microseepage) is predominantly vertical, and that this leakage can be detected and mapped using any of several geochemical and non-seismic geophysical methods. While seismic data are unsurpassed for imaging trap and reservoir geometry, in many onshore geological settings seismic data yield no information about whether a trap is charged with hydrocarbons.

Five critical geologic attributes commonly cited that must be satisfied in order for a prospect to result in an oil or gas discovery are (1) hydrocarbon source rocks, (2) hydrocarbon migration and charge, (3) reservoir rock, (4) trapping, and (5) preservation. While each one of these factors or attributes must be properly developed in a prospect if one is to have a hydrocarbon discovery, there will be no oil or gas discovery without the presence of hydrocarbons in the trap and reservoir. Post-drilling evaluations of dry holes tend to attribute most failures to incorrect structural interpretation and/or unanticipated poor reservoir quality. Only rarely is failure attributed to lack of hydrocarbon charge. One could argue, however, that the cause for most of these dry holes is the absence of significant hydrocarbons, whether that absence is due to lack of charge, poor reservoir quality reservoir, inadequate seal, or lack of closure.

Hydrocarbon microseepage data can provide direct evidence for the probable hydrocarbon charge of the lead or prospect. In order to quantify the reliability of hydrocarbon microseepage data for pre-drill predictions of hydrocarbon charge, published microseepage survey results have been compiled for more than 2700 exploration wells, and comared with subsequent drilling results. These prospects are located in both frontier basins and mature basins, onshore and offshore, and occur in a wide variety of geologic settings. Target depths ranged from 300 meters to more than 4900 meters and covered the full spectrum of trap styles. Prospects were surveyed using a variety of geochemical and non-seismic geophysical hydrocarbon detection methods including free soil gas, integrative soil gas, microbial, iodine, radiometrics, and micromagnetics. Of wells drilled on prospects associated with positive hydrocarbon anomalies, 82% were completed as commercial discoveries. In contrast, only 11% of wells drilled on prospects without an associated hydrocarbon anomaly resulted in discoveries. These results clearly document that hydrocarbon microseepage data – when properly acquired, interpreted, and integrated with conventional exploration data – can reliably predict hydrocarbon charge in advance of drilling.