Understanding Talus Formation from Analysis of Wells Data and Conventional Core Descriptions in a Pinnacle Carbonate Reservoir, Central Luconia, Sarawak

Abstract Talus formations hold potential for hydrocarbon reserves, yet drilling and assessment in these areas are not commonly favoured. Central Luconia's carbonate fields have seen limited talus appraisal. The field of study (Field-X) is one of few fields in here drilled and appraised talus as part of its development drilling campaign. Insights derived from this campaign offer valuable data to address and mitigate various uncertainties on depositional concept, and assess future regional potential. Six wells were drilled during the development campaign in the study area, with two specifically targeting the appraisal of the talus zone. These wells underwent comprehensive logging programs and conventional coring. Pressure data and fluid sampling were conducted to ascertain connectivity between the talus and reef pinnacle, essential for volumetric reserve estimations. Biostratigraphic descriptions of the cores offer valuable evidence for identifying the origin of talus materials, aiding in concluding the depositional concept. Additionally, correlations between field data and regional events, such as sea level variations and wind direction, was studied to strengthen the depositional model. The conventional core biostratigraphy analysis reinforces the depositional theory of talus formation. Core features provide compelling evidence supporting this theory. Talus zones are primarily characterized by skeletal packstone and skeletal wackestone lithofacies, with prominent allochems such as skeletal fragments dominated by red algae, forams, echinoderms, occasional mollusc fragments, and bryozoans. Bryozoa and coral fragments are exceedingly rare, with occasional presence of dark pyritised grains. Physical structures like solution seams and stylolites indicate chemical compaction, leading to decreased porosity. Solution seams often accompany organic cumulates and occasionally clay lenses. Talus originates from eroded carbonate fragments and reef debris, deposited at flanks in deeper waters. Its composition includes non-skeletal particles (carbonate grains, mud, and clasts) alongside erosional skeletal particles (mollusc, foram, algae, echinoderm, coral fragments). Talus generally exhibits lower reservoir quality due to poor sorting and chemical compaction. Absence of sub-aerial exposure limits diagenesis processes altering porosity. Through this appraisal campaign, the presence of hydrocarbon potential in talus formations has been proven. With limited wells drilled for talus appraisal in Central Luconia, this paper serves as a valuable reference for other fields where talus occurs. Offering essential insights and lessons learned, this study provides critical information applicable to all carbonate fields featuring talus. Understanding talus formation is crucial for fostering confidence in future appraisal programs, and optimizing development and production strategies effectively.