engleski

Facies distribution of the Lower and Middle Miocene rock formations in Eastern part of the Drava depression

In the Drava depression, SW part of the Pannonian Basin System (PBS), there is a variety of Lower to Middle Miocene rocks, which were generally formed during sin-rift phase of the basin evolution. These rocks are proven hydrocarbon reservoirs which makes them the valuable object for the detailed study. Input data for the mapping of this Early to Middle Miocene succession is a compilation of the newly acquired observations from outcrops, previous interpretation from deep wells data together seismic data. Integrated approach enabled the construction of a model of facies distribution for rocks that formed during the basin rift and early post rift phase, by associating lithofacies units defined on outcrops and cores to the geophysical measurements in wells and seismic. Five main lithofacies units were defined based on geological data – rockfall breccia along with alluvial fan to debris breccia and conglomerates (1), volcanic and pyroclastic rocks (2), continental conglomerates to marls (3), shoreface conglomerates, sandstones to basinal turbidites and marls (4) and Badenian Lithothamnion limestones and associated shallow water sediments (5). Rock fall and debris breccia and conglomerates are diachronous, formed from Lower to Middle Miocene, marking sin-rift sedimentation, generally presented in well cores. Volcanic and pyroclastic rocks differ in age, from Lower to Middle Miocene and they can be distinguished in well data and on outcrops. Continental conglomerates to marls are predominantly of Lower Miocene age. They are well presented in several locations on outcrops but intend to be reduced in depression and are the most probably only present in lower portion of the thickest successions of sin-rift sediments. Marine shoreface conglomerates, sandstones to basinal turbidities and marls are of Middle Miocene age, presented primarily within the depression and identified in numerous well cores. Badenian limestones and associated shallow water sediments were observed on outcrops in many locations, but they are generally absent in deeper part of depression, occurring only in one well. Since sin-rift rocks are formed in tectonically very active setting and environments from continental to marine, their response on well logs doesn’t have to be repetitive on wells, but rather dependable on the consolidation and secondary processes. Proximal lobe-hummocky to discontinuous seismic facies is attached to rockfall breccia, alluvial fan and debris breccia and conglomerates along half-grabens margins and those directly overlying the Neogene basement. These rocks are varying in thickness from 10 to 500 m. Volcanic rocks are associated to sub-parallel discontinuous seismic facies, and are predominantly present in the western part of the study area with thickness up to 500 m. Lithofacies interpreted on geological data as continental conglomerates to marls and marine shoreface conglomerates, sandstones to basinal turbidites and marls are associated with oblique low amplitude seismic facies and parallel and sub-parallel continuous seismic facies. Further separation of these two lithofacies using seismic is not possible. They are occurring as the main half-graben infill, but with varying thickness from 10 to 500 m. Well data suggest that Badenian limestones and associated shallow water sediments are, if present in wells, probably resedimented in deeper part of the basin and that they occupied positions on drowned basement heights.

syn-rift pahe, Badenian, seismic data, Drava depression, Croatia

nije evidentirano