Abstract
Deep-water turbidites can hold significant amounts of hydrocarbon reserves. One principal element of reserves estimation is accurate hydrocarbon-in-place determination. In this type of reservoir this value is often difficult to obtain as turbidites are primarily composed of thinly bedded shale and sand beds. When shale and sand lamina are below a tool's vertical resolution, petrophysical properties, for example porosity, are averaged across the shale and sand lamina. Also, resistivity measured by conventional induction devices is biased toward the low-resistivity lamina-component, e.g. shale. This creates the classical low-contrast, low-resistivity predicament where quantification of hydrocarbon saturations and net-to-gross is problematic.
Tensor resistivity measurements provide both horizontal and vertical resistivities. The vertical resistivity is more representative of the high-resistivity lamina component, e.g. hydrocarbon-saturated sand. The tensor resistivity measurements are also used to measure large-volume (cubic meters) formation dip and azimuth.
Tensor resistivity and nuclear magnetic resonance data were acquired in this deep-water turbidite field. The data were used to provide a more accurate hydrocarbon–in-place value than was obtainable using conventional resistivity measurements in low- contrast, low-resistivity shaly sand formations and to obtain formation dips and azimuths. A methodology directed at the sand- lamina petrophysical properties was used. Work was performed in a total porosity system, using dry-shale density calculated by comparing nuclear magnetic resonance (NMR) porosity data with the neutron-density porosity in shale.
By comparing the final analysis with core data, it is shown that the integration of tensor resistivity and NMR data accurately computes oil saturations, sand fraction porosity and net sand thickness. There is a large increase in the calculated cumulative hydrocarbon volume over conventional petrophysical approaches due to increased precision and accuracy in the sand-lamina porosity, hydrocarbon saturation, and net sand thickness determination.
