Petrological and organic geochemical insights into the peat-forming environmental settings during the deposition of the early Oligocene Pirin coal deposit, SW Bulgaria

The up to 30-m-thick Brezhani coal seam in the Pirin deposit (SW Bulgaria) is the only coal of early Oligocene age on the territory of Bulgaria, providing important insights into peat-forming environments during that period. The coal seam was investigated using organic petrography, bulk organic geochemistry and biomarker analysis.

Coal is predominantly composed of detrovitrinite with subordinate collotelinite and abundant corpogelinite. Liptinite macerals are mostly represented by alginite and suberinite, and locally resinite. The coal commonly contains semi-inertinized (oxidized) root tissues without apparent thermal influence. The low- to moderate tissue preservation indicates predominant contribution from plants with low preservation potential. Rare resin-impregnated woody tissues argue for peat accumulation within a sparsely forested shrub-dominated mire. Biomarker proxies indicate a contribution from mixed vegetational communities, composed of flowering plants and bald cypresses. A contribution of aquatic macrophytes is also evident.

Variable ash yields and sulfur contents indicate changeable water supply and pH conditions. However, high sulfur contents and ash yields in a large number of the samples denote organic matter accumulation within a topogenous mesotrophic marginal limno-telmatic mire. Plant material accumulated at or very close to the water surface in a predominantly water-logged environment, indicated by strong gelification. The initial oxidizing conditions were quickly shifted to reductive settings due to flooding of the peat surface with possible seasonal dynamics.

Low maturity of the organic matter and limited hydrocarbon generation potential are evident from the low vitrinite reflectance (avg. Ro = 0.52 %), biomarker parameters and Rock-Eval data.