Oil-Base Borehole Image Applications in Thinly Bedded Sand Shale Sequences

Abstract

Micro-resistivity borehole image logs are well-established tools of geologist and reservoir engineers. These data are used for detailed reservoir description, providing high-resolution structural and sedimentological data. For thinly laminated turbidite sequences, they are often the only practical method of determining the distribution of net pay thickness in the absence of whole core data. Additionally, micro-resistivity images are used to help select intervals for formation testing and perforation. The increasing use of oil and synthetic oil-base mud systems to reduce drilling risks and improve drilling efficiency has created an environment that prohibited the use of conventional micro-resistivity imaging devices. Thus, it was imperative to develop a new micro-resistivity imaging technology for oil-based mud systems. This paper summarizes the development and successful application of a new oil-base micro-resistivity imager (EARTH ImagerSM) that brings well-accepted resolution and formation response characteristics of conventional micro-resistivity imaging technology to the non-conductive drilling mud systems. Combining the EARTH Imager with advanced open-hole logging instruments, such as the multi-component induction log (3D ExplorerSM), significantly improves petrophysical evaluation of thinly bedded sand-shale sequences. The interpretation model is built on a combination of high-resolution information from borehole image logs and the 3D Explorer horizontal and vertical resistivity data. These data are used in the Laminated Shaly Sand Analysis (LSSASM) petrophysical model to determine laminar sand resistivity, hydrocarbon saturation, and net sand pay. In our experience, such an approach provides a volumetrically balanced system that is highly reliable for predicting the production potential of an exploration well, a critical step when allocating resources for new development projects. Introduction As shallow-water hydrocarbon producing areas are becoming fully exploited, the frontiers of exploration are being pushed further and further into deepwater. Recent exploration efforts have focused on the deep marine turbidite sands with potentially huge hydrocarbon reserves. Numerous deepwater discoveries have been made in basins around the world as the exploration pace has quickened. The petroleum industry has demonstrated that these deepwater sands are excellent reservoirs capable of sustaining high production rates, thus dramatically increasing the economics for deepwater projects. Consequently, many E&P companies have elected to move into deepwater as rapidly as technology allows. The high-resolution borehole images are one of the most important tools for interpretation of deepwater sediments. The information derived from images is typically used for deepwater channel processes characterization, litho-facies determination, and vertical facies successions (channel stacking pattern). Furthermore, high-resolution borehole image data are routinely used to evaluate thinly bedded reservoirs, especially in the absence of core data. Borehole imaging does not replace outcrop or conventional core information, but in many cases is the glue that links core and outcrop data to the producing field. However, in today's economic environment where outcrop studies and conventional coring is considered an expensive luxury, borehole image data becomes the best tool and in many cases is the only data available for the interpretation of deepwater sediments. The science of borehole data collection and interpretation has been constantly advancing with many exciting improvements in recent years. Prensky (1999) provides an excellent bibliography of borehole imaging. Lovell et al., (1999) and Thompson (2000) document the main developments and applications to present. Lofts and Bourke (1999) detail the quality control necessary for interpretation of such images. The growing popularity of oil-based mud systems has hitherto provided an environment that precluded the use of conventional micro-resistivity borehole imaging technology. Economics and drilling considerations associated with using oil-based mud often outweigh the benefits gained by running micro-resistivity-imaging tools. Consequently, high-resolution analysis of thinly bedded deepwater reservoirs in the absence of core data becomes a major issue.