AAPG Middle East Region Geoscience Technology Workshop

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Advanced Surface Data Logging for Low Resistivity Pay


Open hole logging technology in porous formations has been used with success by testing the resistivity differences between hydrocarbon filled and water/brines in different mineral matrices. In nonconventional tight mudstones formations however, lower resistivity readings are partially due to Cation Exchange Capacity (CEC) factors in complex tight lithologies and because of the nano-sized, non connected pore spaces where kerogen macerals are deposited. Within such non-conventional formations, a part of the non-organic material has the ability of lowering the overall resistivity due to the Cation Exchange Capacity (CEC). We are talking about clays which can be distributed across the formation as plately/laminar material or dispersed at different scale within the formation (ie mudstone). Moreover, in such formations (source rock), the hydrocarbons are thermogenically generated from the kerogen macerals which are trapped in nano-sized pore spaces of these plately clay minerals. Consequently, it becomes impossible to describe the formation as a combination of a « resistivity independant » organic matrix with a low resistivity water phase and a high resistivity hydrocarbons phase. Recent technologies in Advanced Surface Data Logging are now available to propose a comprehensive interpretation workflow able to help solving this issue. It will enhance the open logging interpretation for low resistivity conventional pay zone while it can be the only deterministic logging technique for non-conventional formation such as source rock. These technologies are two-folds: (1) onsite geochemistry analysis of cuttings will identify non organic materials having a Cation Exchange Capacity or potential of lowering the resistivity, (2) advanced method to extract hydrocarbon components from drilling mud coupled with high speed chromatography for extended range of molecular hydrocarbon characterization. The purpose of this paper is to describe these 2 techniques and its application for low resistivity formation across the full spectrum from conventional reservoirs to source rocks.