Diagenetic features recorded in sedimentary rocks within a gas chimney: A case study from the Makassar Strait, offshore Indonesia

Abstract

Methane seeps, prevalent in ocean basins globally, indicate upward methane migration from the subsurface, often evident as gas chimneys in seismic reflection data. The footprint of this methane migration is often indicated by methane-derived authigenic carbonate (MDAC), a product of anaerobic oxidation of methane (AOM). Despite extensive research on MDAC from present-day seafloors and outcrops, understanding methane migration footprints from subsurface rock samples remains limited. Therefore, this study aims to investigate methane migration footprints from subsurface rock samples taken from a proven area of gas migration. This study utilized cutting samples from well XS-01 located in the Makassar Strait, offshore Indonesia. The well was drilled through a gas chimney into Oligocene carbonate reservoirs hosting a substantial methane column (102 m). Analysis of 44 cutting samples involved petrographic examination, Scanning Electron Microscope (SEM) imaging, and X-ray Diffraction (XRD) analysis to discern mineralogical content and diagenetic features signaling the presence of gas. A diagenetic texture called clotted peloidal micrite (CPM) was discovered within foraminifera fossils and around fractures based on petrographic analysis. CPM is a type of MDAC and predominantly occurs in fine-grained, siliciclastic rocks, indicating gas migration. This migration is interpreted to occur: (i) during the early burial stages originating from microbial activity since the Oligocene (biogenic gas); (ii) following matured source rock that reached its peak maturity from the Middle Eocene to the Pliocene (thermogenic gas); or (iii) interference of these two processes. The migration route persists until present day as evidenced by gas chimney and seabed pockmarks identified in seismic reflection data. This study emphasizes the importance of subsurface rock samples, such as cuttings, in uncovering gas migration footprints. Especially, where seismic data is unavailable or could not image fluid flow features. In addition, this study also provides a new perspective on diagenesis along methane migration route, complementing most of the research that is primarily focused on reservoir diagenesis.