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.

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