Tectonic Control on the Formation of Cleats in the Coal Beds of the Sajau Formation, Berau Basin, Northeast Kalimantan

Abstract

Coal seams of lignite-to subbituminous rank of Pliocene-age from the Sajau Formation of the Berau Basin are characterized by the presence of a natural fracture system, including cleats. Often, these opening-mode fractures consist of two orthogonal sets (face and butt cleats), both almost perpendicular to the bedding. This paper describes the distribution of orientation, spacing, and aperture of cleats formed in the seams of the Sajau Formation. All observations and measurements were conducted at macroscopic, mesoscopic, and microscopic scales. The butt cleat mean orientations are NE-SW and NW-SE, whereas the face cleat mean orientations are NNE-SSW and NE-SE, and the cleat dip is at a high angle of 75° to the north. The angles between the orientations of these cleat sets are nearly 90°; that is, they are orthogonal. The spacing of the macroface cleats is between 9.52 and 14.46 cm (averaging 11.61 cm), while the spacing of the butt cleats is between 2.3 and 11.3 cm (averaging 5.35 cm). The mean aperture is 0.54 mm. In contrast, the mean spacing of the mesoface cleats is 3.09 mm, and the aperture measurements of these cleats range from 0.0478 to 0.0741 mm (averaging 0.0549 mm). The results obtained from outcrops and X-ray CT (computed tomography) scan clearly indicate that the NNE-SSW face cleat orientations are strictly parallel to the elongation of the main tectonic structures in the studied area. Their origin may be explained by their relationship with local tectonic stress (the maximum principal stress, σ1, was horizontal). Partial least square analysis of data collected from cleats and faults in the area indicates that a power law distribution exists between the cleat characteristics (spacing, density, and aperture), and the distance of the faults (R 2 = 0.56). Cleat formation in the Sajau Formation was mainly controlled by the mechanical response to tectonic stresses, which generated a unique cleat network in the coal. Based on the SEM photography, it is concluded that the development of microcleats in the Sajau Formation resulted from both endogenic processes and tectonic activities as indicated by the change in the shape of the cleats, from straight line to curved shape and branching (intersection of multiple cleats that form a conical network at the end).

Structure Analysis Gunung Benau Karst Area by Using mSTA and FFD, North Kalimantan, Indonesia

“…which is dominated by this orientation. Meanwhile, the predominant orientation of regional geological structures in the Berau Basin is NW-SE [6,7,8,9]. This region is also surrounded by the NW-SE trending Maratua and Mangkulihat Faults.…”

Kalimantan hydroclimate variability since the last glacial period

“…The sub-basin is geologically subdivided into the Lati syncline and Rantaupanjang anticline with NNW-SSE fold axes (Suwarna and Hermanto 2007). The sub-basin was formed by extension and subsidence during the Middle-Late Eocene, which created wrench faults and resulted in the appearance of major NW-SE oriented structures (Subroto et al 2005;Sudradjat and Hamdani 2018). Economically important lithologies filled the Berau sub-basin from the Cretaceous Telen and Benggara Formations, followed by shales with tuffs of the Eocene-Oligocene Sembakung Formation, Late Oligocene-Early Miocene carbonate sequences of the Selior and Tabalar Formation, and Plio-Pleistocene fluvio-deltaic and shallow marine deposit of Domaring and Sajau Formation (Krisnabudhi et al 2020).…”