Petrofacies and reservoir quality of the Tortonian Sandstone reservoirs, north Kutei Basin, East Kalimantan

The Anamta field, the focus of this study, is a prolific hydrocarbon discovery in the offshore Kutei Basin, having yielded over 900 million barrels of oil equivalent (mmboe) from the Miocene fluvial-deltaic sandstone reservoirs. One fifth of the total current producing wells drain gas from the deepest under-optimized Basal Tortonian Sandstones (BTS). Increased burial with depth and changing toward distal sandstone facies within the overall progradation package in the study area have always been considered detrimental to the flowability of deep Tortonian reservoir target. The objectives of this study are to integrate the parasequence sets and detrital mineral composition of the Tortonian sandstone in order to build petrofacies units and articulate them with reservoir quality characterization based on the key physical properties of the sandstones, with specific focus placed on the BTS as the object of this study. Biostratigraphic markers in the wellbores were tied to seismic data to define depositional systems tracts of highstand (HST) 01, 02, 03, lowstand (LST), transgressive (TST), and HST 04 and 05. QFL analysis of 77 thin sections and XRD reveals a shift from sublitharenite in the basal part to feldspathic litharenite and litharenite toward the upper Tortonian sandstones. Qm-F-Lt, Qm-P-K, and Qp-Lv-Ls ternary plots indicate recycled-orogen provenance. By integrating the systems tracts analysis, classification and provenance with facies detailed descriptions from core data, three distinct petrofacies were defined: (1) Basal, (2A) Middle, and (2B) Upper Tortonian Sandstones that hereinafter referred to as BTS, MTS and UTS. Principal component analysis (PCA) upon these petrofacies identified permeability, siderite, and kaolinite are the key principal components (PC), while K-means clustering confirmed Petrofacies-1 is a high-quality reservoir with permeability ranging in 28-211 milli-Darcy (mD) and porosity between 12%-20%. It is characterized by sublitharenite composition, minimal siderite/kaolinite, and fine- to slightly medium-grained size. These findings highlight Petrofacies-1 as a promising deep target for future delineation and exploration. • Petrofacies definition is vertically based on parasequence sets of deltaic setting, detrital mineral composition, paleo climate, sub-regional relative sea-level curve, and sedimentation rates. • Petrofacies-1 (Basal Tortonian Sandstones) is below the Forced Regressive Shelf Break (FRSB), while the other two Petrofacies (2A and 2B) are positioned above the FRSB. • The Petrofacies-1 is dominated by polycrystalline quartz (Qp) of sublitharenite, differently with mixed feldspathic-litharenite sandstones above the FRSB. • Eigenvalues suggest three main Principal Components: PC-1, PC-2, PC-3 are dominated by permeability (mD), carbonate and clay mineral cementation, respectively. • K-Mean clustering analysis of the PC-1 vs. PC-2 and PC-1 vs. PC-3 suggests that the main objective (Petrofacies-1) is mainly grouped in the Cluster-1 with permeability of 28 - 211 mD, • This Cluster-1 is dominated by medium to well sorted, very fine to fine grained delta-front sandstones with no siderite and kaolinite, and still possesses a porosity of 0.12 - 0.20 at depth of 9,700’ - 10,160’ below mud line (bml).