ESTIMATING SOURCE-ROCK ORGANIC RICHNESS FROM WELL DATA IN PARTS OF WESTERN NIGER DELTA, NIGERIA

Abstract

A study was conducted to estimate source-rock organic richness from well logs. Source-rocks are measured by the amount of organic matter (TOC) present in sediments. TOC is often estimated in the laboratory by analysis of sample data. These laboratory methods are limited by sample collections and movements from the field to the laboratory. As a result of difficulties in core and side wall sampling, many of the boreholes are without TOC data, an important parameter for source rock studies. In the Niger Delta, since the traditional methods of the laboratory are used, an attempt is therefore made to determine TOC values from geophysical logs because of their advantages over laboratory methods. The geophysical data obtained from Shell Petroleum and Development Company (SPDC) in parts of western Niger Delta are in-situ and are continuous along the borehole. Five wells available for the analysis are Pologbene-001 (with GR, sonic, resistivity and density logs), Okwefe-001 (with only GR, resistivity and density logs), Abraka-001 (with GR and resistivity logs only), Umutu-002 (with GR, sonic, resistivity and density logs) and Ameshi-001 (with GR, sonic, resistivity and density logs). The wells intervals chosen were controlled by the availability of rock-eval TOC results for the ages. Bivariate regression analysis on well logs was used to relate the well logs with TOC. Source rocks consists of three components namely, the organic matter, matrix and fluids. Bivariate regression analysis involves finding a direct relationship between two variables, in this case, the log values along the borehole and the TOC data (rock-eval) while keeping the matrix and the fluids constant. Four of the five wells (Pologbene-001, Okwefe-001, and Umutu-002) were used for the derivation of the equation, while the fifth well (Abraka-001), were used to test the results. Three steps were followed in the analysis. The first step was crossplot of rock-eval data against the well log values at corresponding sampling points. The second step was removal of matrix and fluids effects masking the relationships between TOC and well logs. The third step was crossplot of filtered TOC and well log data. The results show straight lines passing through all the points with correlation coefficient R2 equal to 0.9488 and 0.9632 for the gamma ray the sonic logs respectively. The equations of the lines were used to calculate TOC for Eocene Section. For Abraka-001 (an independent well in the region), at sampling points 10261 ft (3128 m), 10425 ft (3178 m), 10470 ft (3191 m), 10920 ft (3328 m), 10990 ft (3350 m) and 11010 ft (3356 m), the calculated TOC were 2.203514 wt %, 2.277716 wt %, 2.044993 wt %, 2.190023 wt %, 2.119195 wt % and 2.213633 wt % respectively. These values correlate with the rock-eval (laboratory TOC) values of 2.16 wt %, 2.38 wt %, 1.74 wt %, 2.26 wt %, 1.88 wt %, 2.05 wt % at the same sampling points respectively. It is thus concluded that TOC can be obtained from well logs.