Aurelia Liliana Stan1 and Adriana Raileanu2
Search and Discovery Article #10046 (2003)
*Adapted for online presentation from poster session presented at the AAPG Convention, Salt Lake City, Utah, May, 2003.
1Geophysicist, Romanian Oil Corporation Petrom-S.A.,Geological Exploration Research and Design Center, Bucharest, Romania (firstname.lastname@example.org)
2Sedimentologist, Romanian Oil Corporation Petrom-S.A.,Geological Exploration Research and Design Center, Bucharest, Romania (email@example.com)
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The study area is located in the Romanian part of the Central Moesian Platform. The focus of this paper is the recognition of the main Permian-Jurassic fault systems in order to define the major events: tectonics, eustacy, and sedimentation.
The basement faults are responsible for igneous intrusions and extrusions. Permian-Triassic magmatism is associated with a W-E rift area, generating horst-graben structures. Three Jurassic intracratonic basins were recognized: a basin characterized by syndepositional faults and thermal subsidence; a strike-slip basin; and a fossil rift basin, with en echelon external faults.
In the first basin, deposition took place as a result of subsidence and eustacy interaction. In the second basin, a strike-slip fault system developed, with two main faults. The direction of sedimentary influx is identical to the direction of the block movement. The source area for this basin is located on the uplifted horst, which was cannibalized. In the fossil rift basin, the direction of the Jurassic sediment influx is opposed to the block movement. Active bi-directional erosion was generated externally along the normal fault escarpment of the fossil rift shoulder. Internally, an active depocenter shifted to the source area.
Middle Jurassic offshore distal sandbars, littoral bars, and delta-front and fan delta represent the prospects for hydrocarbon fields. The main Upper Jurassic prospect is represented by carbonate shelf margin, with diagenetic control on the pore system.
The hydrocarbon fields are distributed asymmetrically, and they are mostly encountered in the strike-slip basin.
Figure 5. Interpreted seismic profile across Draganu-Fauresti area, showing formations of Getic Depression thrust over Moesian Platform and the highly eroded Fauresti Horst with scarp splay fault of the strike-slip system SS, tilted and rotated blocks within strike-slip system SS2, and syndepositional normal faults.
Figure 7. Interpreted seismic profile across northern eroded faulted horst (Mamu Field); different structural types are present: normal fault block of northern Triassic horst (FN1), strike-slip zone showing positive flower structure type (transpression).
Figure 9. Interpreted seismic profile across the highest southern Triassic horst (Ciuresti) superposed over an older uplift (from Carboniferous magmatism); basinward there are normal faults with high throws; the seismic shows high Triassic sediment input controlled by subsidence.
Figure 11. Intrepeted seismic profile across Oporeolu-Ciuresti-Crampoaia area, crossing the two strike-slip fault systems and Permian depocenter with coarse-grained facies; Ciuresti-Mogosesti area is controlled and partitioned by SS1 and by antithetic (North Ciuresti) and synthetic (South Ciuresti) faults in en echelon manner.
The study area is located in the Central Moesian Platform between Oltet-Teleorman rivers, the Danube River, and the Peri-Carpathian Fault (Figure 1). The Moesian Platform presents four major sedimentary cycles of sedimentation. Permian and Triassic deposits belong to the second cycle, while the Jurassic deposits to the third one. Locally, erosional hiatus or non-deposition characterizes the sedimentary record. In the NW part of the area, Middle Jurassic siliciclastic facies presents an argillaceous depocenter, which is the seal for the Aalenian and Bajocian sandstone reservoirs. Fractured dolomite deposits (Figure 2) represent Tithonian reservoirs.
The focus of this article is the recognition of the main Permian-Jurassic fault systems in order to define the connection between the major events in basin evolution: tectonics, eustacy, and sedimentation. The Jurassic hydrocarbon fields lie in the northern part of the study area. The well data were integrated into the seismic lines highlighting the seismic markers, formation succession, and fault systems, as well (Figure 3). The analysis of seismic data was achieved using seismic stratigraphic techniques, the identification and correlation of regional stratigraphic markers using reflector terminations, sequence boundaries defined by unconformities, and their correlative conformities.
The Permian-Triassic basement faults present N-S and E-W trends. The latter faults are responsible for igneous intrusions and extrusions associated with pyroclastic rocks along Craiova-Optasi Uplift, due to Hercynian and Cimmerian Orogenesis.
In the north of the Moesian Platform, the Permian-Triassic magmatism is associated with an extensional rift area, along W-E trending fault system, generating horst-graben structures. The components of the structures are: Mamu-Mitrofani-Spineni northern horst; Fauresti-Iancu Jianu and Optasi-Ciesti-Buzoiesti median horst; Strejesti-Oporelu-Mogosesti and Braniste southern horst, structurally attached to Slatina-Ciuresti highest horst (Figures 12, 13, and 14).
Three types of the Jurassic intracratonic basins were recognized (Figure 15). The first (1) is the southern-central basin characterized by normal syndepositional faults and thermal subsidence (Corabia, Studina, Plopii-Slavitesti, Diosti, Boianu, and Malu Mare structures). The second (2) basin is strike-slip type and is located between SS1 and SS2 strike-slip faults (Ciuresti, Priseaca, Oporelu, Strejesti, Fauresti, Mamu, and Draganu structures). The third (3) type is a fossil rift basin, with external en echelon, normal faults (Tatulesti, Braniste, and Tomsanca structures).
1. Jurassic deposition in the southern-central basin took place as a result of subsidence and eustacy interaction. The source areas were the former Triassic uplift (Leu, Corabia, and Harlesti structures), and the main transport trend was SE-NW, and locally NE-SW.
2. In the second basin, the strike-slip fault system presents 2 km displacement. It consists of:
- Two main faults, a sinistral one (SS1) and a dextral one (SS2).
- Normal splay faults (Fx1, Fx2, Fx3, Fy).
- Antithetic and synthetic en echelon faults (Mamu, Fauresti, and Ciuresti areas)
- A normal transfer fault and a horsetail splay fault (Varteju and Golumbu areas).
The direction of sedimentary influx was the same as the direction of the uplifted block movement. The source area for this basin was located laterally on Iancu Jianu, Fauresti and Mamu uplifted horst, the latter one being cannibalized to the north (escarpment fault with conglomerates facies in Mitrofani-Dumitresti area), and to the south towards the deeper Draganu basin.
3. In the third, fossil rift basin - Spineni, the direction of the Jurassic sedimentary influx was opposed to the block movement, externally generating active bi-directional erosion along the normal fault escarpment of the fossil rift shoulder, and internally an active depocenter shifted to the source area.
Jurassic Depositional Sequences and Tectonics
Neglecting Jurassic post-tectonic events, the restored geometry of depositional sequences and their definition on logs, in terms of facies models, allowed us to predict and correlate the main regional events in basin evolution (Figures 16, 17, 18, and 19). In Toarcian, III order composite sequences were induced by long-term uplift conditions, which also determined, in early Aalenian, the development of the HST succession, only. In Middle Bajocian, active subsidence led to the absence of the basal and middle parts of the III order depositional sequence (Figures 18 and 19).
In Callovian, due to regional extensional conditions, the Moesian Platform tilted and rotated, and an important discontinuity occurred as a maximum flooding surface. This surface was used as a marker in restoring of the depositional profiles. Other III order composite sequences are of Oxfordian and Tithonian ages. These are composed by higher frequency IV and V order sequences, as minor transgressive-regressive cycles. The Tithonian sequences are defined as two autochthonous carbonate wedges in a LST regime. To the south, the Tithonian carbonate wedges have an aggradational geometry, strongly influenced by the constant subsidence rate superposed on a general tendency of rapid sea level lowering. To the north, the distal pelagic carbonate facies shows progradational geometry on the by-pass, faulted (in the strike - slip system) slope, with a low rate of sedimentation.
Due to the strike-slip deformation in the NW, high subsidence prevailed. The uplift influenced the sedimentation in the NE and SE, where subaerial erosion resulted in intraformational hiatus or re-sedimentation (Figures 19, 20, and 21). In late Bajocian, late Callovian, and late Tithonian, the strike-slip system was more active, while the influence of the syndepositional normal faults decreased.
To illustrate the distribution of the linkage and contemporaneous depositional systems, two examples were chosen: late Bajocian siliciclastic Falling Stage Systems Tract and late Tithonian carbonate autochthonous wedge Lowstand Systems Tract.
The Middle Jurassic depositional systems are represented by coastal fluviatile domain, littoral and offshore bars, strand plain, delta system, shelf, shelf margin, slope, fan delta and basin (Figure 20). Offshore distal sandbars, littoral bars, and delta front and fan delta represent the prospects for Iancu Jianu, Fauresti, Spineni, Oporelu, Bacea, and Ciuresti hydrocarbon fields.
In Late Jurassic, the depositional systems consisted of carbonate shoals, banks or reefs on the internal and external shelf, a marginal shelf, faulted slopes, and basin (Figure 21). The main prospect is represented by Tithonian carbonate shelf margin with diagenetic control on the pore system.
The defining of the major events in the basin evolution of Central Moesian Platform (Figure 22) led to the following conclusions :
- Based on older and new considerations, Permian-Jurassic successions may be interpreted in terms of intracratonic extensional basins followed by subaerial erosion and strike-slip deformation in the northern part of the study area; in the southern-central areas, conditions for thermal subsidence of the basin prevailed.
- The hydrocarbon fields are distributed asymmetrically. The fields producing from the Triassic and the Dogger are located in Malu Mare, Iancu Jianu, Negreni; South Ciuresti, Fauresti, Spineni, Simnic, Ghercesti, Circea, Malu Mare, and Ciuresti fields are producing from the Dogger, only, while North Ciuresti and Barla fields are also producing from the Tithonian.
- It is worth noting that most of the Jurassic commercial hydrocarbon accumulations are encountered in the strike-slip basin, and they are controlled by antithetic and synthetic, en echelon faults and other typical secondary features.
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Tari, Gabor, Oprea Dicea, Joe Faulkerson, Georgi Georgiev, Svetlozar Popov, Mihai Stefanescu, and Gary Weir, 1997, Cimmerian and Alpine stratigraphy and structural evolution of the Moesian Platform (Romania, Bulgaria), in Regional and petroleum geology of the Black Sea and surrounding region: AAPG Memoir 68, p. 63-90.