Customized interpretation workflow for simultaneous assessment of unconventional shale-gas reservoirs and conventional reservoirs using seismic and log data

Abstract

The same geophysical and geologic data have been interpreted for both unconventional shale-gas and conventional sandstone reservoirs simultaneously by framing and using a customized workflow. Customization was needed because of property differences between the two types of resources. Unconventional shale reservoirs differ significantly from conventional reservoirs in depositional environment, diagenesis, physical properties, and production techniques. Magnitudes and patterns of reservoir properties (e.g., velocity, resistivity, radioactivity, porosity, permeability, etc.) of shales differ significantly from those of sandstone/carbonate reservoirs. In a conventional play, we generally search for intervals with relatively low gamma, high resistivity, and low neutron porosity, whereas in shale gas we search for intervals with very high gamma, relatively higher resistivity than lean shales, low velocity, and very high neutron porosity. Production from shales is largely due to horizontal drilling, hydrofracturing, and stimulations. The shale-gas reservoir properties are governed mainly by organic richness and its thermal maturation. Organic material has significant impact on the elastic/petrophysical properties, which are manifested in log and seismic responses. Thus, identification and mapping of shale-gas reservoirs is based mainly on mapping of total organic carbon- (TOC) rich zones. The customized workflow was developed for identification of shale-dominant intervals and organic-rich-zones within them by integrating log and seismic data. Shales generally show low-amplitude signals, and internal reflection configurations are not easily identified. Seismic data conditioning and seismic attributes, e.g., phase and perigram, were applied for internal geometry mapping. Once shale-gas markers were identified, mapping of shale boundary, depth, thickness, and areal extent was done similarly to conventional plays. Sweet spots were identified by generating and analyzing log-property volumes like impedance, sonic, resistivity, and neutron porosity. Natural fractures, which are helpful in designing well trajectory, were mapped through geometric seismic attributes like curvature and dip azimuth. This study has enhanced geologic understanding and, hence, mappability of shale-gas plays along with conventional plays. The customized workflow is applicable in all stages of shale-gas-play exploration and development.