Low Resistivity Pay Evaluation, Case Study: Thin Bed Sand-Shale Lamination Reservoirs, Peninsula, Malay Basin

Abstract

Abstract Thin laminations reservoirs of sand and shale, which contains hydrocarbons was determined in some reservoirs at Malay Basin, but not identified as hydrocarbon bearing, or hydrocarbon-in-place are not properly quantified. The net pay and hydrocarbon saturation assessment are a challenge in thin-layered and shaly sand reservoir, with low resistivity and low contrast log response. However, there are several productive reservoirs with similar characters. In thin lamination shaly-sand, fine grained of sand, silt and clay were distributed within the hydrocarbon bearing reservoirs. The character of these reservoirs will be determined as a low net reservoir and reduce the apparent resistivity. Most of the hydrocarbon saturation is difficult to be assessed by conventional log interpretation and leads it as low pay zone or water bearing reservoirs. Meanwhile, the higher cost of exploration and production will determine these as un-commercial reservoirs and the opportunity of this reservoir will often be missed. The factors that affect the low resistivity low contrast include bed thickness, grain size, mineralogy, structural dip, clay distribution and water salinity or any combination of the above will affect the well logging instrument to measure lower resistivity response than would ordinarily be expected inside a good productive formation. This response of low resistivity-low contrast is due to the low vertical resolution of the conventional logging tools, which can not measure thin-bedded pay zones. One of the case studies which is presented in this paper, the sequence of sands reservoir from Peninsula, Malay basin has low net reservoir due to high shale contain and low resistivity, almost close to shale resistivity. The well testing and lab analysis prove this interval as oil bearing in the present work we conduct the evaluation of low resistivity-low contrast for thin bed lamination of shaly-sand reservoir to optimize the hydrocarbon saturation quantification. Some approaches were performed to determine shale distribution and saturation hydrocarbon. Thomas-Stieber method was used to determine the shale distribution in the reservoirs and its properties. The horizontal resistivity (Rh) and vertical resistivity (Rv) was acquired on the next closest well to calibrate the resistivity model which using Hagiwara macroscopic anisotropy approach. And Waxman-Smitts method was performed to calculate the final fluid saturation. The hydrocarbon saturation results in thin-bedded reservoirs in the current study was improved this interval was defined as pay zone by perforated it and it produces 600 oil bbl/d and 0.2 mmscf/d. This method can be used for others wells and other reservoirs which have a similarity of geological forces to improve the resource assessment, especially to assess hydrocarbon saturation in pay thin-bedded reservoirs.