Clay mineralogy and organic matter of Upper Miocene to Pliocene mudstones of the Kampungbaru Formation, Lower Kutai Basin, Indonesia

The Kampungbaru Formation is considered a potential source rock for petroleum in the Lower Kutai Basin, Indonesia. An integrated approach of organic petrography, total organic carbon (TOC), Rock-Eval pyrolysis, XRD analysis, and clay mineralogy of fifteen outcrop samples obtained from the Upper Miocene – Pliocene Kampungbaru Formation was conducted. The sediment is characterized by heterolithic, laminated mudstone and interbedded sandstone. Sandstones form thin layers and lenses with predominant lenticular to flaser bedding. The mudstone samples of the Kampungbaru Formation generally have total organic carbon (TOC) content between 0.06 to 8.76 wt.%.  The organic matter consists mainly of vitrinite (71 - 79.6 vol.%; avg. 75.3 vol.%), liptinite (16.90 - 27.4 vol.%; avg. 22.15 vol.%), and inertinite (1.5 – 3.50 vol.%; avg. 2.55 vol.%). T_max values of these Upper Miocene – Pliocene mudstones are below 435 °C and vitrinite reflectance ranges from 0.27 ± 0.05 %Rr to 0.33 ± 0.05%Rr, indicating an immature stage in the diagenesis of kerogen. The pyrolysis data (HI vs. T_max) show that type III kerogen dominates organic matter with HI values of < 200 mg HC/g TOC and would be expected to generate gas.  All of the organic matter in the samples is inferred to derive from terrestrial plants. Semi-quantitative XRD results of the mudstone samples in the studied section include varying proportions of clay and non-clay minerals. The bulk mudstone samples in the section are dominated by clay minerals (33.4%–63.0%, avg. = 47%), quartz (23.8%–60.5-%, avg. = 37%), and pyrite (6.0%–32.4%, avg. = 16%). The < 2 μm clay fraction consists of kaolinite, illite, mixed layer illite/smectite (I/S), and chlorite. Kaolinite is the most abundant clay mineral encountered in the studied Kampungbaru Formation. Kaolinite content is influenced by the sedimentary lithology setting. Most sediment deposited in the Mahakam Delta likely originates from erosion by the antecedent part of the Mahakam River.