This study examines an important channel sand reservoir in the Junggar Basin, which has a reservoir quality that varies greatly in the vertical and horizontal directions due to the presence of tight sandstones deposited under different sedimentary conditions. Studies confirmed that the high-energy sandstone deposits have good porosity and permeability based on the core data. The channel sand is 20 m to 30 m thick in the Qigu formation and can be divided into three to four channels. Identifying and characterizing the high-energy sandstone "sweet spots" was the objective of this case study. In order to find the sweet spot in this reservoir, high-definition oil-base mud microresistivity image logs were acquired in five exploration wells. The image logs made it possible to identify the different kinds of sandstone reservoirs based on sedimentary structure and texture. Parallel bedding, deformed bedding, cross bedding, and massive bedding were all identified. The paleocurrent energy was from high-to-low and from bottom-to-top in a single channel sand body. The core and production data validated the reservoir qualities, which were identified from the sedimentary features. The channel sand was divided into three to four multistory sand bodies based on image logs. As a result, the typical channel-levee system—composed of massive bedding, cross-bedding and parallel bedding features—was revealed. The massive bedding was mainly at the bottom of the single channel system, surmounted by high-angle cross-bedding, and then finally the parallel bedding intervals. The paleocurrent energy changes from high-to-low vertically within the single channel system. Based on the open-hole and image logs, two types of sweet spots were identified: massive bedding sandstone and high-angle cross-bedding sandstone. These two types were deposited in a high-energy environment which formed the potential sweet spot in the channel complex. The massive bedding intervals, which are mainly in the channel bottom, when cemented by calcite or clay, will become tight sometimes. The high-angle cross-bedding sandstone and uncemented massive-bedding sandstone were the potential target zones for the horizontal well. This case study will show the interpretation of sedimentary features in the channel sand reservoir based on high-definition image logs, to provide a comprehensive understanding of the geologic features that differentiate reservoir quality. The identification of cross-bedding in the channel sand provided a new way to predict possible tight sandstone sweet spots that can help to design horizontal wells.

Since the introduction of the first micro-electrical imaging tool in 1986, wireline resistivity images have proven to be an invaluable tool for geological and petrophysical formation evaluation in wells drilled with conductive water-base drilling mud (WBM). However, until recently, wellbore images acquired in non-conductive mud had been met with some less success due to poor borehole coverage, relatively low image resolution and electrical artefacts. In 2014, an OBM-adapted imaging tool was introduced. The new tool was designed to provide improved resolution and borehole coverage as well as geological representativeness of the images. From an operations perspective, the tool sonde and hardware were designed to increase robustness and ease of logging for field engineers, and to improve operational efficiency and reduce rig time in consideration of high spread rates for deep-water drilling rigs and the overall high costs of deepwater wells. The sonde design with two sets of pads supported by spring loaded arms allow both logging down and logging up of the tool to minimize logging time. Unlike previous imaging tools, pads are applied to the formation using spring load and not pad pressure, in order to minimize stick-slip of the tool. Pads are fully gimballed, are free to tilt, and rotate around the pad axis to enable maximum contact with the borehole wall. As for the measurement physics, a high frequency current is sent into the formation which reduces the non-conductive mud electrical impedance. Amplitude and phase of this current are measured and used in the processing to create an electrical impedivity measurement. In order to cover the full range of formation resistivities, two frequency ranges are used. After acquisition, a "composite" processing technique is used in which amplitude and phase measurements from the two frequencies are processed to generate a final impedivity image that is a function of formation resistivity and dielectric permittivity. The case study presented in this paper is an Oligocene-Miocene age deep-water turbidite deposits on the passive margin of West Africa, and comprises a complex of channels and sheet sands with localized intense faulting, and tilting due to salt tectonics and diapirism. The high-resolution image enabled highconfidence classification of geologic features. The variety of geologic features ranges from fine-scale laminations and syn-depositional micro-faults with displacement of a few centimeters to variable-scale injectite features and erosive surfaces. Also, a wide variety of formation textures that represent turbidite channel and levee facies are observed, and include coarse-grained basal conglomerates, rip-up clasts and large clay clasts, debrites, dewatering and flame structures, dish structures, internal injectite structures, pyrite nodules/streaks, and deformed facies. The high resolution image can be used for a wide range of quantitative image analyses such as net pay computation, textural attribute extraction, as well as other quantitative and semi-quantitative interpretations. Today, with more than 13 case studies in West Africa and more than 250 worldwide, the image quality from this new formation imaging technology shows a great deal of improvement over previous generations of non-conductive mud imagers. The ultrahigh-resolution images from the new imaging service enables a wide spectrum of interpretations that can be directly incorporated to enhance the reservoir model and reduce geological and petrophysical uncertainties.

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Shale gas reservoirs in the LongMaxi Formation, Sichuan Basin, are the biggest proven shale gas reservoirs in China. The natural fracture system plays an important role in the shale gas exploration and development. Both high-angle calcite-cemented fractures and low-angle bedding fractures were observed in the core and resistivity image logs but the relationships between these two kinds of fractures is still not clear. Identifying and characterizing the natural fracture system in the shale gas reservoir were the objective of this case study. The borehole resistivity image logs were acquired in several shale gas wells in southeast Sichuan Basin. Weobserved a spiky resistivity log response in some intervals of Longmaxi Formation. Image logs reveal that all these intervals have low-angle conductive features. Based on the crosscutting relationship of bedding and fractures, these features are conductive bedding fractures, which cut the high-angle resistive fractures in the formation. Both conductive and resistive bedding fractures are identified in the image logs and core data. The geochemical spectroscopy logs in this interval show low pyrite volume, which indicates the conductive bedding fractures are not filled by conductive minerals. It could be an effective pathway of fluid flow. The natural fractures can be divided into three types: low-angle conductive bedding fracture, low-angle resistive bedding fracture, and high-angle resistive fracture. The bedding fractures and high-angle resistive fractures are mainly in the brittle zone of the LongMaxi Formation. We observed that the high density of the conductive bedding fractures will appear spiky on the resistivity logs, which will cause high horizontal permeability. Multiwell correlation shows that there will be more conductive bedding fractures in areas where the shale gas reservoir was undergoing tectonic stresses, which could have a negative impact on the shale gas enrichment. The bedding fractures were formed after the high-angle resistive fractures. Some bedding fractures were cemented by calcite, while some are still open. The identification of bedding fractures will help to understand the complex natural fracture system in the tight shale gas reservoir. The conductive bedding fractures will greatly enhance the horizontal permeability and will influence the fluid flow dramatically.

Zubair Formation is one of the key producers in North Kuwait; however, the reservoir complexity and hydrocarbon movement along with pressure depletion always poses challenge for determining the perforation and completion strategy to optimize the production. Zubair Formation is broadly divided into three parts e.g. upper, middle and lower, the upper and lower units are of utmost importance for the current study. An integrated approach was adopted utilizing the high-resolution borehole image outputs; which has not only helped in identifying thin bedded reservoir zones but also facilitated the understanding of detail reservoir geology and sand dispersion. Integrated formation evaluation and workover design is always the key to sustain the production and it becomes even more important when the reservoir is highly heterogeneous in nature and coupled with declining pressure trend. Therefore, an innovative methodology was necessary to address the uncertainties. High resolution borehole images were utilized to determine the sand count, which can detect even the thinnest reservoir layer in the formation. Heterogeneity analysis was also performed to understand the relative sorting of the different reservoir units; sorting has a direct relation with reservoir permeability and thus reservoir productivity. High resolution sedimentary analysis was performed to understand the detailed sedimentology using the borehole image derived dip data; cross bedding types were identified which provides fair idea about depositional energy condition along with depositional environment. All the high-resolution inputs were integrated with openhole logs and volumetric results, which led to a clear deterministic picture of the reservoir, based on which crucial decision was taken. This integrated approach was adopted in three deviated well sections in Zubair formation, which has facilitated in improving the well performance. Detail sedimentary analysis and cross bedding typing in multiwell helps in fine tuning the sand dispersion in the reservoir model; which in turn found to be helpful for deciding future well locations.