![]()
Summary Well logs from several fields in Assam, India were analyzed to determine the feasibility of a 3C seismic development study. One of the challenges in the area is to identify sand-rich regions within a structural geologic setting. We find that representative reservoir sands (e.g., Barail) have an identifiable response; in particular, higher S-wave velocities compared to their surrounding sediments. Synthetic seismograms generated from the logs suggest that these reservoir sands may be detectable with PP and PS seismic techniques.
Introduction The Assam area is located in northeastern India (Figure 1), in a tectonically active area. The NE-SW trending basin has similarly oriented faults which control some of the hydrocarbon traps. Reservoirs are mostly in anticlinal structures and some subtle stratigraphic traps (Wandrey, 2004). Current oil production in the region is mainly south of the Brahmaputra River and north of the Naga Hills. The historic Digboi oilfield, in this area, was discovered in 1889 (Dasgupta, 2007). Important hydrocarbon-producing strata include: the Paleocene Langpar, Eocene Sylhet and Kopili, the upper Eocene-Oligocene Barail Group, and Miocene Tipam and Girujan facies (Wandrey, 2004). The main target in this study is the Barail Group, which was deposited in a deltaic environment. The Barail units can contain interbedded coals, sandstones, and shales.
Elastic properties and rock properties were analyzed using well logs from two wells in the Makum (Well A) and Deohal (Well B) fields, to evaluate the feasibility of usingmulticomponent seismic to enhance interpretation in the area. The motivations for acquiring multicomponent (especially, converted P-to-S) seismic data are several-fold, including: improvement of the P-wave sections (via multicomponent filtering techniques), developing new structural details (faults, compartments, closures) using PS images, assisting with defining new stratigraphic features and providing some large-scale lithology (e.g., sand versus shale) information, and help in providing information about fluid distributions.
Rock Properties Quite productive reservoirs can be found in the Barail and Tipam sandstones. Their permeabilities range from less than 7 millidarcies to 800 millidarcies, and porosities up to 30%. An example of a suite of logs, from well B, is given in Figure 2. Note that there are distinct S-wave velocity (Vs) anomalies in the Barail sands, with Vs generally increasing with sand quality. This is an importantobservation for the future use of converted (P-to-S) waves as the PS reflection coefficient is a function of the change in Vs across the interface. In general, good sands are indicated by a lowered SP, low gamma ray (~50 API), 30% porosities, lowered densities, resistivities between 10 and 20 ohms, and high Vs. The sands generally plot (on a Vp versus Vs chart) below Vp/Vs=2 and below the mudrock line. the vertical. A density-porosity log was calculated usingdefault sandstone parameters, with a matrix density of 2.65g/cc and a brine density of 1.09 g/cc. Slight crossover ofthe neutron and density porosity at the top of the Barail4 and 5 intervals on well B, indicates a gas cap might be present. However, Mathur et al. (2001) note that interpreting porosity logs can be complicated, at least at the Eocene level.
Site News