AVO detection of gas-producing dolomite trends in nonproducing limestone

Abstract

The AVO technique has been successful in hydrocarbon exploration for gas-sand reservoirs such as those in the Texas Gulf Coast, the North Sea, and West Africa, but there are limited examples of successful AVO application in carbonate exploration. Commonly in carbonate reservoirs, dolomitized zones have better reservoir quality than limestones. Because dolomite has a lower Poission's ratio than limestone, AVO might be able to detect dolomite trends. In this paper, we use AVO analysis to detect gas-producing dolomite in the nonproducing Black River limestone. The available data are: (1) NMO-corrected CDP gathers for three 2D lines (lines 1, 2, and 3); (2) log data for five wells: A (a gas dolomite well @ CDP 376) and B (a dry limestone well @ CDP 114) on line 1; C (gas dolomite well @ CDP 486) and D (dry limestone well @ CDP 410) on line 2; and E (dry limestone well @ CDP 148) on line 3. The main zone of interest is the high impedance Lower Ordovician Black River carbonate (Figures 1–3) at depths greater than 8000 ft. This reservoir is believed dolomitized by hydrothermal fluids moving through the host limestone along fractures associated with faults. Well-log data show that the dolomitized reservoir porosity averages about 5%. The limestone is very tight with average porosity less than 2%. Figures 4–8 show the well-log characters of the Black River carbonate in the available wells. Figure 1. Line 1 showing Black River reflector. Figure 2. Line 2 showing Black …