D35 Field: Integrated Approach to Manage Challenging Reservoir Characterization Study and Impact of the Revised Geological Model for Ultimate Recovery

The structural complex of D35 Field presents a variety of challenges to geoscientists and engineers. Located in the Balingian Province offshore Sarawak, it was discovered in 1983 and has become one of the major oil-producing fields in the area (Figure 1, Location Map). The structural complexity, resulting in compartmentalization of the reservoir is attributed to the collision and wrenching episodes between the Central Luconia Province to the north, and the onshore Tinjar Province and the Rajang Fold-Thrust Belt to the south (Mazlan Madon & Peter Abolins, 1999). In addition, Swinburn (1994) reported that the western part of the Balingian province, where D35 is located, was subjected to a folding, faulting, and erosion, thus affected the sedimentation processes throughout reservoir sands of Cycle I to Cycle IV. Because of this complexity, for the past 25 years, the interpretation on depositional setting of the reservoir sand has changed few times. In 1984 when with limited data; it was interpreted as coastal plain with<br>fluvial channel reservoir. In 1988, a ‘Mississippi’ delta interpretation was introduced before it was replaced by fluvioestuarine environment in 1994. In 2003, the interpretation was revised back to a fluvial environment with a series of stacked and isolate channels envisaged. The current study in 2009, however, has established a coastal sedimentary setting as the environment to be more realistic based on these new findings are the results of the comprehensive and integrated undertaking in Geology, Geophysics,<br>Petrophysics, and Petroleum Engineering. This paper focuses on the prolific sands of Cycle II and Cycle III reservoirs. The geological studies encompass application of sequence stratigraphy in well log correlations, and detail core analyses include sedimentological facies descriptions, ichnofacies study, coal petrology, and biostratigraphy. The Cycle II depositional setting (Figure 2) has been established as the coastal and related estuarine deposit rather than stacked channel deposits previously interpreted. The prograding units are sheet with thick and widespread as shown in figure. Cores recovered from the unit show evidence of marine influence such as burrows of Ophiomorpha and sedimentary structures typical to the coastal deposits. Analysis of the trace fossil assemblages demonstrates a variety of sub-environments, from a marine influenced lower coastal plain to a fully marine environment. The Cycle III depositional setting (Figure 2) has been established as the coastal longshore bars deposited in a transgressive system tract. Log correlations show the stacking pattern of the parasequences as retrogradational (Figure 3). As in Cycle II, cores from the unit show evidences of marine influence, as opposed to fluvial, interpretation used in previous models. Sandstone thicknesses derived from geostatistical inversion work show northwest-southeast trending sand bodies (Figure 4) consistent with the longshore bar environment interpretation. These new findings translate to widely distributed sands in the established Cycle II and Cycle III, in contrast to the channelized and stacked sand patterns in the previous model. These have given positive impacts on the current hydrocarbon volumes and potential exploration prospects. The findings also support the assessment of leads in exploration potentials in Cycle II and Cycle III.