Overview and Reinstatement of Some Geophysical Interpretations in the Eastern Sava Depression

The eastern part of Sava Depression is bounded on the south with the Sava River and on the north with Psunj Mountain, Požeška and Dilj Hills. In this part thicker Neogene-Quaternary sediments than in the western part were found, and there is almost never reached thermal maturity sufficient to catagenesis. Therefore, after the first seismic survey and exploratory drilling, a new additional research never continued. The overview and assessment of the reconstruction of the part of geophysical measurements was prepared in the eastern part of the Sava Depression. In some previous interpretations rocks between the Upper Pannonian and Upper Pontian are described as result of deltaic systems, with the clinoforms. Regional deposition at the time of the Early Pannonian indicates the beginning of significant changes in the environment in Croatian part of the Pannonian Basin System (CPBS), with a distinct domination of turbidites. Clinoforms are possible results of especially psammitic detritus deposited toward the margins of the depression, and partly in the direction material transport. In the Early Pontian regionally depositional environments started to be local. In the Quaternary can be expected the existence of the delta or alluvial fans.


Introduction
The article presents the overview of a small part of the previous research and results in the eastern part of the Sava Depression and has made rating reinterpreted data and models.In the study area has not been determined oil or gas fields, but in several wells oil and gas occurrences are found.For example, Ţgaljić (2003) stated evidence of oil on the surface in locality Paklena and in the village Baćindol.Potential reservoirs are situated in the Pannonian and Pontian sandstones (Ivanić Grad, Kloštar Ivanić and Široko Polje Formations) and in the Middle Miocene sediments of Prkos and Prečec Formations.In some previous interpretations rocks between the Upper Pannonian and Upper Pontian are described as result of deltaic systems.However, regional deposition model in the Early Pannonian indicates that turbidites start to predominate in the Croatian part of the Pannonian Basin System (CPBS), what lasted to the end of Late Pontian.
The Mining-Geology-Petroleum Engineering Bulletin, 2015

Geography
The eastern part of Sava Depression is bounded on the north with Psunj Mountain and Poţeška and Dilj Hills, and in the south with the Sava River.The area that covers approximately 325 km 2 is covered with very rare network of 2D seismic profiles (25 profiles) where the distance between them is over 5 km (Figure 1).

Review of previous researches in analysed area
Profiles are recorded in the period from 1978 to 1990.In the period from 1955 to 1962 13 exploration wells are drilled.The 3 of them reached basement rocks (informal unit "Temeljno gorje"), 3 have reached Middle Miocene sediments (Prečec Formation), and 7 are finished in the Lower Pontian sediments (Kloštar Ivanić Formation).In 2 were found in oils and gas shows, in 1 of oil, and in 3 of gas (Table 1, Figure 1).

Table 1. The wells located in the analysed area
The Mining-Geology-Petroleum Engineering Bulletin, 2015

Neogene and quaternary deposition through megacycles in analysed area and lithostratigraphy
The Neogene and Quaternary sedimentary complex in the studied area can be subdivided into three major megacycles (Velić at al., 2002;Velić, 2007).The oldest (1st) megacycle represents the Prečec Formation.The sedimentary complex is extremely heterogeneous, comprising breccias, conglomerates, sandstones, siltstones, shales, marls, igneous rocks and limestones.The Prkos, Ivanić-Grad, Kloštar Ivanić and Široko Polje Formations belongs to 2 nd megacycle and are represented mostly by a monotonic marl/sandstone sequence.The 3 rd megacycle is represented by the Lonja Formation, composed mostly of unlithified sediments, i.e. sands, gravels, loess and some lignite.

Litostratigraphic review of the exploration area
Based on lithological units, conditioned by tectonic movements, sedimentary development is spatially divided into three parts (Figure 2): 1. South-western part with the wells 7, 12 and 13 includes the complete lithostratigraphic development, ending into basement rocks; 2. The north-eastern part includes the wells 10, 11 and 6.In this area are missing Lonja, Široko Polje and Kloštar-Ivanić Formations.Ivanić Grad and Prkos Formations are drilled; 3. The northern and central parts include the wells1, 2, 4, 5 and 9.In this part sequence of upper part of Široko Polje Formation are missing.Ivanić Grad Formation is not drilled.The Mining-Geology-Petroleum Engineering Bulletin, 2015

Description of lithostratigraphic formations
Lithological units are described in detail by formations.Prečec Formation represents the first Neogene-Quaternary megacycles in Sava Depression, including Badenian and Sarmatian (Figure 2), and locally possible deposits of Lower Miocene.The sedimentary series comprises breccias, conglomerates, sandstones, clay and limestones.Sediments of this formation are proven only in the south-west (wells 7, 12 and 13) and north-east (wells 6, 10 and 11) of the analysed area.
On the Sarmatian deposits overlapping sediments of the Early Pannonian or Prkos Formation, bounded by e-log markers Rs7 and Rs5 (Figure 2).In the southwestern part of the area are deposited calcite-rich marls (wells 7, 12 and 13).Sedimentation in the northeastern part (wells 10 and 11) differentiate from the one in the southwestern part, because in NE thin sandstone layers with oil shows appear.Sedimentation in the Late Pannonian (Ivanić Grad Formation) indicates the beginning of significant changes in sedimentary environments, with the dominant transport of turbidity currents.Formation is bounded by e-log markers Rs5 and Z' (Figure 2).In the south-western part of depression (7, 12 and 13 wells) marl and sandstone accumulated.At the north-eastern part of the depression (10 and 11 wells) marl with high percentage of clay is described.Sandy component is very fine-grained and inserted, like lamina, in marls.
Sediments of Kloštar Ivanić Formation are bounded by markers Z' and Rφ (Figure 2).They are composed of finegrained sandstone and marl.On the all analysed area fine-grained sandstones and marls are proven, but in the north and central part deposits of this formation are thicker.
Široko Polje Formation (Late Pontian) can be observed on the south-western, northern and central part of the area, and the north-eastern part is not deposited.It is bounded by markers Rφ and ά (Figure 2).On the south-western part, small occurrences of gas are registered.Sandstones and clays alternated with marl.In the northern and central part of the area upper part of formation is not deposited.On the northeastern part formation are missing.E-log marker ά separates deposits of the Lonja Formation and base (Figure 2).Lonja Formation includes weakly consolidated or unconsolidated sediments of Pliocene, Pleistocene and Holocene, like clays, sands and low carbonised coals.In the south-western part sediments are composed of sands and clay with rare layers of coals and sandstones deposited in freshwater conditions.In the northern and central part sediments become thinner.

Reinterpretation of some previous geophysical results in analysed area, problems and uncertainties
Seismic profile AA 'is connected with the profile B-B'.They do not continue directly, but its direction is on the southeast of area (Figure 3).Near the profile in Figure 2 are wells 12 and 13 distant about a kilometre (precisely well 12 about 1700 m, and 13 approximately 1000 m) (Figure 3, Figure 4).In the Late Pontian deposits, it is possible to interpret delta environments from the EK diagrams of well 12 (Figure 5).Within the Široko Polje Formation (top to bottom) alterations of sandstones and marls are visible in the geological column.In the same column, sandstones are visible at a depth range of 800-1000 meters and marls are visible below sandstones as well.Furthermore, it should be mentioned that the source rocks in Sava depression generally belong to Prkos and Prečec Formations.Here they are drilled in wells 7, 12 and 13, in NW part.In the earlier studies (Final Report, SE Sava 2D, 2011), organic-geochemical analysis of surface rock samples are done.Samples contain considerable amount of organic substance which is thermally immature.Source rock has no generative possibilities.Assuming continuous increase of thermal maturity with depth, in this area the entry into ''oil window '' can be expected at a depth of more than 2500m.
The Mining-Geology-Petroleum Engineering Bulletin, 2015

Conclusion
In the south-eastern part of Sava Depression thinner Neogene-Quaternary sediments than in the western part were found, where almost never reached thermal maturity, necessary for catagenesis is.This overview is given for the part of geophysical and well measurements in the eastern Sava Depression.In previous interpretations it was determined that clinoforms are not visible on the seismic profiles, as well as the fluvial channels and deltas.However, based on the seismic interpretations connected with existing knowledge of the regional geological evolution and structures in the CPBS it could not be confirmed possibility of deltas existence in Pannonian and Pontian.It especially cannot be stated as unique geological feature on such relatively small area inside the CPBS, as it is the eastern Sava Depression.Here the Upper Miocene sediments were also dominantly deposited from turbidity currents.Delta and alluvial fans can expect locally in the Late Pontian, Pliocene and Quaternary, what can be indicated, e.g., at the e-log, like in well 12 (Figure 5).On north -eastern part of analysed area Široko Polje Formation missing.On northern and central parts also upper part of this formation are missing.For this small area it is almost impossible.Lithotypes and thickness of individual sedimentary events in previous interpretation (Završni izvještaj, JI Sava 2D, 2011), should be definitely re-analyzed.
Wrong interpretation had been previously done.

Figure 1 .
Figure 1.Location map of analysed area with a network of 2D seismic profiles and exploration wells

Figure 2 .
Figure 2. Formal lithostratigraphic and chronostratigraphic units, E-log markers and average thickness (thicknesses are not in scale) in the analysed area (schematic representation of units between south -western, northern and central parts and north-eastern part)

Figure 3 :
Figure 3: Location map of analysed area with profile A-A'-B-B' and Well 12 (red)