A Fresh Look at Thin-Bed Prospectivity within Conventional Shallow Water Clastic Plays—Implication for Significant Bypassed Pay

摘要

Thin-bedded sands in shallow water clastic deposits are widely neglected as exploration targets simply due to conventional wisdom that focuses more on the“charismatic”blocky clean sand packages. In addition, the lack of a strong, characteristic motif on the log response and complexity in quantitative characterization of these thinly laminated sequences leads to even less attention to such sequences as potential targets. This can have significant impact on the certainty of realistic reserve calculation in exploration and development planning in regions such as Southeast Asia where these thin-bedded sands are a considerable portion of the stratigraphic column. This study has two phases; in the first phase, a novel technique is presented to identify and quantify thin sand laminations using borehole images and core. Both borehole image and core data is converted to binary data representing shale and sand facies. Then these images are used to quantify the total thickness of thinly-laminated sands on the basis of specific cutoff values. In the second phase, a comprehensive analysis of a local present-day analogue determines the possible hydrocarbon storage capacity of thin-bedded deposits. This is accomplished through an area/volume approach. In this approach, after locating the pertinent locality within the depositional environment that can be considered as the equivalent to the subsurface section under study, the sand volume vs. area of thin-bedded heterolithic facies is computed. This is used to estimate the storage capacity of these facies at the field-scale. Additionally, the storage capacity is compared to the capacity of channel- fill sands as conventional targets in the same setting. Results of this study show that the new technique using high-resolution borehole images can successfully identify and quantify thin-bedded sands, and results have an excellent match with core data. Also, the core results confirm that these deposits have sufficient reservoir quality (porosity and permeability), particularly for gas production. This work demonstrates that, contrary to conventional belief, the storage capacity of thin-bedded sands can be significantly high and sometimes may even exceed that of blocky channel or mouth-bar sands. This implies that substantial quantities of pay are continuously bypassed in basins where these sequences are expected.