Interpreting the Depositional Facies of the Upper Gharif Fluvial System UsingOil-Based Mud Images: A Case Study in Central Oman

摘要

During the early stages of developing Oman's A field, water-basedrnmud (WBM) was used to drill all wells. With this mudrnsystem the wells suffered borehole stability problems,rnespecially in the friable paleosols (fossil soils) of the Gharifrnformation. One of the advantages of drilling with WBM wasrnthe ability to run high-resolution microresistivity images.rnIn the second development stage of the field, oil-basedrnmud (OBM) was used in order to reduce the severe boreholernwashouts. However, the benefits of microresistivity boreholernimages were lost.rnWith the development of the oil-based mud imager toolrnand its deployment in the A field, key geological andrnpetrophysical information is provided for further successfulrnfield development.rnThis paper explains a new methodology for geologicallyrnevaluating the complex upper part of the Gharif formation byrnintroducing integrated data from different sources. Using thernelectrical oil-based mud images integrated with acousticrnimages, core data and openhole logs, the facies distributionrnand typing of the Permian Gharif fluvial system are interpretedrnover the field.rnNine depositional facies were identified using the oil-basedrnmud images: sand channel, heterolithic channel, abandonedrnchannel, splay/chute, debris flow/sand flat, floodplain/swamp,rntidal flat/intertidal, marine shales, and shallow marinernlimestone facies.rnThe Upper Gharif stratigraphic interval is subdivided intornchannel belts separated by thick floodplain shales. Thernpaleocurrent analysis based on these subdivisions illustratesrnthat, in the Upper Gharif, the large spectrum of trendsrnobserved for some layers is the result of deposition withrnalmost no accommodation space (meandering channel),rnwhereas other layers show clearer trends indicating possible deposition under higher energy (stream channel type). Thernvalue of the oil-based images has been its ability to delineaternthe channel belt trends and sand prediction for infill drilling.rnThe identification of these facies and their trends is criticalrnfor reservoir evaluation and in field development. However,rnrecent infill drilling results in the Upper Gharif show that sandrnprediction remains the key challenge to successfulrndevelopment.