Porosity Development Controlled by Deep-Burial Diagenetic Process in Lacustrine Sandstones Deposited in a Back-Arc Basin (Makó Trough, Pannonian Basin, Hungary)

Abstract

Deeply buried Pannonian (Upper Miocene) siliciclastic deposits show evidence of secondary porosity development via dissolution processes at a late stage of diagenesis. This is demonstrated by detailed petrographic (optical, cathodoluminescence, fluorescence, and scanning electron microscopy) as well as elemental and stable isotope geochemical investigations of lacustrine deposits from the Makó Trough, the deepest depression within the extensional Pannonian back-arc basin. The analyses were carried out on core samples from six wells located in various positions from centre to margins of the trough. The paragenetic sequence of three formations was reconstructed with special emphasis on sandstone beds in a depth interval between ca 2700 and 5500 m. The three formations consist, from bottom to top, of (1) open-water marls of the Endrőd Formation, which is a hydrocarbon source rock with locally derived coarse clastics and (2) a confined and (3) an unconfined turbidite system (respectively, the Szolnok and the Algyő Formation). In the sandstones, detrital grains consist of quartz, feldspar, and mica, as well as sedimentary and metamorphic rock fragments. The quartz content is high in the upper, unconfined turbidite formation (Algyő), whereas feldspars and rock fragments are more widespread in the lower formations (Szolnok and Endrőd). Eogenetic minerals are framboidal pyrite, calcite, and clay minerals. Mesogenetic minerals are ankerite, ferroan calcite, albite, quartz, illite, chlorite, and solid bituminous organic matter. Eogenetic finely crystalline calcite yielded δ13 C V − PDB values from 1.4 to 0.7‰ and δ18 O V − PDB values from –6.0 to –7.4‰, respectively. Mesogenetic ferroan calcite yielded δ13 C V − PDB values from 2.6 to –1.2‰ and δ18 O V − PDB values from –8.3 to –14.0‰, respectively. In the upper part of the turbidite systems, remnants of the migrated organic matter are preserved along pressure dissolution surfaces. All these features indicate that compaction and mineral precipitations resulted in tightly cemented sandstones prior to hydrocarbon migration. Interconnected, secondary, open porosity is associated with pyrite, kaolinite/dickite, and postdates of the late-stage calcite cement. This indicates that dissolution processes took place in the deep burial realm in an extraformational fluid-dominated diagenetic system. The findings of this study add a unique insight to the previously proposed hydrological model of the Pannonian Basin and describe the complex interactions between the basinal deposits and the basement blocks.

Thermal Conductivity of Sediments from Well-Logs and its Application to Determine Heat Flow Density in the Pannonian Basin, Hungary

“…In the shaly sediments the dominant clay minerals are smectite, illite/smectite, illite and chlorite, and additionally, kaolinite is also present as subordinate component (Viczián, 1992(Viczián, , 2002. The detrital minerals in the sandstones are quartz, feldspar, dolomite, small portion of calcite, muscovite, illite and chlorite (Thamóné Bozsó et al, 2006;Király et al, 2019;Laczkó-Dobos et al, 2020). The quartz content varies between 50% -80%.…”

Structural Controls on Basin- and Crustal-Scale Fluid Flow and Resulting Mineral Reactions

“…Although burial normally decreases porosity, sedimentary rocks can undergo dissolution processes at depth, which may lead to an increase of secondary porosity. A detailed petrographic and geochemical study by Laczkó-Dobos et al [34] of the deeply buried Upper Miocene lacustrine sandstones of the Pannonian Basin (Hungary) allows reconstructing their diagenetic evolution. They analyze core samples from wells located at different positions within the Makó Trough to reconstruct the paragenetic sequence of three formations that include open-water marls and confined and slope-related unconfined turbidites.…”

“…In the Makó Trough system, the marls are hydrocarbon source rocks and the uppermost turbidites reservoirs, while the lower (confined) turbidites have no reservoir potential. Laczkó-Dobos et al [34] describe how strong compaction, cementation, and mineral precipitation preceded hydrocarbon migration, while secondary porosity by dissolution developed under burial conditions during late diagenesis due to the circulation of external fluids in an open system. The authors also discuss a hydrogeological model involving basement blocks and basin deposits.…”