What’s new in exploration

Frontier exploration areas can offer large rewards, but obstacles to acquiring 2D regional seismic can prevent making the best decisions in a timely manner. Even if the obstacles can be overcome, a 2D seismic program is costly. Acquiring conventional gravity and magnetic data provides a preliminary investigation of basement structures and sediment thickness, but no information about the reservoir.

A new tool. Optimal placement of seismic can mean the difference between relinquishing a block and drilling a discovery. Stress Field Detection (SFD) represents a new generation of airborne geophysical surveys that can identify areas of reservoir potential within the sedimentary column, enabling the focusing of seismic programs and G&G resources on realizing that potential. SFD deliverables include a report that identifies and ranks areas for trap and reservoir prospectivity. Interpretation results may be gridded to create a prospectivity map, and integration with other data sources ensures that SFD results are calibrated to existing G&G data.

The SFD system utilizes quantum-scale sensors to detect gravity field perturbations induced by terrestrial stress energy variations, primarily in the horizontal plane. Significant subsurface discontinuities (anomalies) are inherently associated with, and dependent on, subsurface principal stresses (Bell, 1996; Zoback, 1998). As a consequence, the discontinuities will distort stress fields, resulting in a unique in-situ stress pattern. In addition to local effects, principal horizontal stresses define migration pathways, reservoir orientation and fluid expulsion (Baranova et al., 2011; Zeng et al., 2004).

The subsurface geological condition required for SFD to detect gravity field perturbations, due to stress variations in the horizontal direction, is the occurrence of a structural and/or stratigraphic change (interface) with sufficient difference in elastic properties. An important source of elastic variations is the presence of trapped fluids (oil, gas or water). Other sources include faulting/fracturing, over-pressure, major lithological changes and basin boundaries.

Generally, major discontinuities will evoke a distinct SFD response. For instance, if a dry rock body is in contact with a fluid-saturated rock, the shear stress at the interface will be reduced significantly, because fluids cannot support shear, yet the normal component of the stress remains continuous (Walley and Field, 2005). In general, the variation of shear component in reservoirs results in the redistribution and orientation change of the stress fields.

The SFD survey system is completely self-contained within the survey aircraft, and utilizes 22 sensors (six primary, eight secondary, and eight R&D), flying at an altitude of roughly 3,000 m and a speed of approximately 500 km/h. A standard survey is normally flown in a grid pattern and is designed to detect anomalies with a linear extent of 2 km to 20 km. Data are acquired at 2,000 samples/second, and the output is displayed as voltage (V) vs. flight time (seconds). Anomalous areas, which are identified during the interpretation process, are assigned a relative ranking, indicative of the trap and reservoir potential.

In the fall of 2012, PEMEX conducted an initial SFD survey in onshore and offshore areas of the Gulf of Mexico region. The survey focused on two objectives: 1) a “blind” test of the SFD system over areas with significant, proprietary geological and geophysical (G&G) information; and 2) identifying new prospective areas in the region. The survey area is geologically complex and covers the Salina del Istmo, Salina del Istmo Deep Gulf portion, Reforma Akal Pilar, and Macuspana basins, as well as the Sierra de Chiapas area and the Yucatan Platform. The sediments of these basins are terrigenous, carbonates, and, in some areas, there is the presence of salt bodies.

The project was completed, and NXT’s recommendations were delivered at the end of 2012. This was followed by an extensive PEMEX integration study, that showed a significant correlation between the recommended SFD anomalies, and both the known oil fields and seismically identified structures. Additionally, SFD anomalies were noted in new areas undergoing initial stages of exploration, where the petroleum system is well-established.

NXT identified 72 anomalies within the boundaries of the surveyed area, ranked by prospectivity. Out of these 72 anomalies, 16 were ranked as first order, 37 were ranked as second order, and 19 were ranked as third order. Only the first and second order anomalies were recommended for further geological and geophysical investigation, while the third order anomalies were recommended for additional SFD coverage.

Editor’s note: Major portions of this column were referenced from: Escalera, Jose Antonio, et. al., “Application of stress field detection (SFD) technology for identifying areas of hydrocarbon potential in the Gulf of Mexico region,” Next Generation Oil & Gas Summit Latin America, Cartagena, Colombia, July 23–25, 2013.  

LITERATURE CITED

Baranova, V. et al., “Integrated geomechanical reservoir characterization approach to study migration and accumulation of hydrocarbons in Llanos basin, Colombia,” presented at the AAPG International Conference and Exhibition, Milan, Italy, Oct. 23-26, 2011.
Bell, J. S., “In situ stresses in sedimentary rocks (Part II): Applications of stress measurements,” Geoscience Canada, vol. 23, no. 3, pp. 135-153.
Walley, S. M., and J. E. Field, “Elastic wave propagation in materials,” Encyclopedia of Materials: Science and Technology, pp. 1-7, 2005.
Zeng, L., C. Tan and M. Zhang, “Tectonic stress field and its effect on hydrocarbon migration and accumulation in Mesozoic and Cenozoic in Kuga depression, Tarim basin,” Science in China, Ser. D Earth Sciences, vol. 47, Supp. II, pp. 114-124, 2004.
Zoback, M., “Scientific Drilling into the San Andres Fault and Site Characterization Research: Planning and Coordination Efforts,” Final Technical Report, U.S. Department of Energy, Aug. 30, 1998.