Depositional Environment Model of Main Jurassic Sand in Ubadari Field, Eastern Indonesia

Abstract

Reservoir presence is one of key risk in Ubadari Field. Understanding reservoir lateral extent and reservoir geometry are required to predict the range of gas in place for preparing further appraisal and/or development plan. The aim of the study is to construct robust sedimentology and depositional description of Main Jurassic Sand in Ubadari Field. An integrated approach based on core and image log data were conducted to build reliable depositional model. The study was accomplished by performing full examination from core data to define sedimentary facies and facies associations. Since the core data coverage is limited, integrating and calibrating core with image log data are important to interpret facies association for un-cored reservoir interval. Image log was heavily analyzed to get paleocurrent direction. After completing sequence stratigraphy evaluation, field scale depositional environment maps were constructed to defined reservoir lateral extent and geometry. Five main depositional environments are defined, which are floodplain, fluvial, foreshore, shoreface, and transition offshore depositional environments. Sandstone is mainly found as fluvial, foreshore, and shoreface deposits. Sandstone that was deposited as an amalgamated fluvial channel has relatively limited lateral extent with typical channel shape geometry. In the other hand, foreshore and shoreface sandstone are interpreted to be extensively distributed across the field as a sheet-sand geometry. Although the majority of reservoir is extensively distributed across both wells, several facies changes are defined from Ubadari-1 to Ubadari-2 well location, leading to potential lateral reservoir heterogeneity, especially from different depositional environment. Any available data must be incorporated to construct reliable subsurface reference case including image log. Integrated core and image log evaluation is highly valuable for building robust interpretation of sedimentology and depositional system, especially when core data coverage is limited. Understanding depositional model of Main Jurassic Sand in Ubadari Field at the early stage could allow a full-field development acceleration.