Abstract
Reservoir characteristics analysis in the onshore Cawthorne Channel (CAWC) oil field, Niger Delta is here presented. The aim of the research was to assess reservoir properties and their relationships. A suite of geophysical logs comprising gamma ray, resistivity, neutron and density logs from eight wells were used in the analysis. Three reservoirs sands were delineated and linked across all eight wells. The litho-stratigraphy correlation section revealed that each of the sand units spreads over the field are differs in thickness with some units occurring at greater depth than their adjacent unit, that is possibly an evidence of faulting. The results show volume of shale values range from 11% to 17% indicating that the fraction of shale in the reservoirs is quite low. The total porosity of the reservoirs ranges from 0.22 to 0.39 indicating a very good reservoir quality and reflecting probably well sorted coarse-grained sandstone reservoirs. The permeability of the reservoirs ranges from 288 mD to 1250mD and this suggests good reservoir horizons. The hydrocarbon saturation of the reservoirs ranges from 0.59 to 0.71 indicating that the proportion of void spaces occupied by water is low consequently high hydrocarbon production. Sand-shale lithology was calculated, with sandstone volume decreasing with increasing depth, while shale volume increases with depth. Porosity and permeability showed decreasing trend with depth for both sandstone and shale units in all wells with few exceptions. This could be as a result of low compaction by overburden pressure from overlying rocks. Plot of lithology versus depth reveals that shale lithology increases with depth, while sandstone decreases. Lithology versus porosity plots show an inverse relationship between permeability and shale volume and direct relationship between permeability and volume of sand. Lithology versus permeability shows that permeability and shale volume have an inverse relationship whereas permeability and volume of sand have a direct relationship. Permeability decreases exponentially with decrease in porosity in rock matrix made up of intercalation of sandstone and shale. The modelled equation of permeability and porosity is given by K = 0.053e32.934Ф. This implies that in the absence of core and well-log data, permeability can be estimated using only porosity data. The results of this work can be used as an exploration tool for the identification of prospective areas and also for feasibility studies during an appraisal activity.
Highlights
Formation evaluation is used to understand the geology of the wellbore at high resolution and to estimate the producible hydrocarbon reservoir
Shale lithologies cause the deflection of resistivity to the far left due to its high conductive nature. It obvious that for every reservoir, porosity is generally high, signatures for volume of shale are low, water saturation are low while permeability are high as seen in the curves
Our results show that shale porosity decreases with increase in depth [12,13,14,15,16]
Summary
Formation evaluation is used to understand the geology of the wellbore at high resolution and to estimate the producible hydrocarbon reservoir. Once formation evaluation is performed on the reservoir, it is crucial to pay attention to the location of the possible reservoir zone, determination of fluid type (gas, oil, water) present in the pore space, saturation level, and the mobility of the fluids across the connected pore space of the rock. To better achieve such information, it is important to have a good understanding of porosity (total, primary, effective porosity), water saturation computation, pay thickness and selection of cut offs. Various well logs which include gamma ray, neutron, spontaneous potential, resistivity and density logs were analyzed and interpreted in order to define lithologic units of prospective zones, differentiating between hydrocarbon bearing and non-hydrocarbon bearing zone(s), and to investigate the relationship between the petrophysical properties
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