Page 218 - Geochemical Remote Sensing of The Sub-Surface

Light hydrocarbons for petroleum and gas prospecting 195

excellent agreement with published soil gas and reservoir data (Jones and Drozd, 1983;
Nikonov, 1971). A single oil source is predicted at Filo Morado, in agreement with the
known oil field. Much gassier soil-gas data is noted over the Loma de La Lata Field,
where there exists an oil field underlain by a gas to gas condensate reservoir. However, a
very striking change to fairly large magnitude oil-type compositional anomalies occurs
directly over the northwest portion of the Loma de La Lata Field where the Quintuco oil
reservoir is the only known producing horizon. This change in composition from oil to
gas condensate signatures over the Loma de La Lata Field occurs across a permeability
pinchout at depth, which controls the updip limits of the deeper gas condensate reservoir.

High Island area, Gulf of Mexico

A marine hydrocarbon seep-detection survey was completed over High Island Blocks
A-152 and A-198 and surrounding areas in the Gulf of Mexico on 22-23 April 1988 (Fig.
5-33a. This study, consisting of 399 km (239 miles) of sniffer data, was conducted
aboard the RV GYRE by Texas A&M University in conjunction with Exploration
Technologies Inc. using a marine hydrocarbon analytical system originally designed by
Gulf Oil Corporation for use on the RV Hollis Hedberg. Light hydrocarbon data were
collected continuously along seismic lines of interest from a water-sampling system
towed about 9 m (30 feet) above the bottom of the sea floor. A total of 87 km (52 miles)
of gridded data (259 analyses) were completed over Block 152A and a total of 51 km (31
miles) of gridded data (129 analyses) were completed over Block A-198. Samples were
taken at 3-minute intervals giving an approximate sample spacing of about 450 m (1500
feet)
Anomaly compositions are plotted on a marine crossplot in Fig. 5-33b for
comparison with the calibration crossplots in Fig. 5-24. Three regional profiles are
presented in Fig. 5-34 to show the magnitude variations along the survey lines.
Survey tracks, as shown on Fig. 5-33a, include a 90 km (54 miles) regional south-
north line which extends from Block A-198 to Block A-321 in the High Island South
extension. The results from this regional line, plotted in Fig. 5-34b, provide both a
calibration data set over the known gas fields and a background data set which extends
between the two gridded blocks. As shown by Fig. 5-34b, background values are
observed in Blocks A-237, A-224 and A-223, where concentrations are about 100 nl/1
methane, <0.70 nl/ethane and <0.50 nl/1 propane. These concentrations correspond with
typical backgrounds found in previous studies in the Gulf of Mexico (Mousseau and
Williams, 1979. The largest magnitude anomalies observed on this entire survey are also
noted on this regional line (Fig. 5-34b), where it crosses the centre of Block A-268 and
traverses the major trend of the known gas-producing fields. Within this producing trend,
methane values exceed 500 nl/1, ethane ranges from 1-5 nl/1 and propane rises from 0.50-
1 nl/1. In addition, iso-butane and normal butane reach a combined total of about 1 nl/1
in anomalies associated with these known gas fields.