Discovery of a new source-rock interval within the Pantokrator Formation, Ionian Zone, western Greece: Insights from sulfur speciation and kinetics analyses

Abstract

The Ionian Zone in western Greece constitutes a Mesozoic sedimentary basin with petroleum potential. Numerous rock intervals with source, reservoir, and seal properties, along with several surface oil seeps have been identified. These rocks are linked to the northern portion of the Ionian Zone in nearby Albania, where similar rocks host oil and gas fields. This study describes a newly discovered source rock interval, the “Pantokrator Shales”, which lies within the Pantokrator Carbonates Fm, between the Rhaetian platform dolomite (“Pantokrator Dolomites” Fm.) and the lower Liassic platform limestone (“Pantokrator Limestones” Fm.) sequence. This interval is an important source rock for the petroleum system of the Ionian Zone in western Greece and Albania. The samples were acquired from outcrops and subsequently examined by programmed pyrolysis and organic petrography, as well as for sulfur speciation. The Pantokrator Shales Formation is comprised of ∼30 m of organic-rich shales alternating with less organic-rich light gray and thin-grained dolomitic sandstones and dolomite beds. The results of organic geochemical and organic petrographic analyses suggest high amounts (up to 38.68 wt% TOC) of Type I and mixed II/II-S organic matter with high generation potential (S1+S2 is up to 269.79 mg HC/g and HI of up to 790 mg HC/g TOC). Selected intervals analyzed have large amounts of labile organic sulfur. Pyrolysis Organic Sulfur (POS) ranges between 1.22 and 4.87 wt% on a whole-rock basis, with Sulfur Index (SI) values ranging from 116 to 174 (minimum SI for Types IS and IIS = 100), suggesting that the kerogen has already entered the oil window and generated liquid hydrocarbons in accordance with reflectance values measured on vitrinite (%VR: 0.60–0.65) and/or solid migrabitumens. Widespread conditions that favored the deposition and preservation of high amounts of oil-prone organic matter (including vulcanization processes of organic matter) existed locally in the basin during the Late Triassic to Early Liassic. This study showed that the stratigraphic position, the lateral and vertical extent, the organic matter quantity and quality, as well as the sulfur (S) content of the Pantokrator Shales, are comparable to intervals in nearby Albania, Sicily and the Italian peninsula. The high clastic input that was recorded may be associated with a regional shift in climatic conditions towards more seasonably controlled wetter conditions, as inferred from adjacent areas.