2. Key Laboratory of Tectonics and Petroleum Resources, Ministry of Education, China University of Geosciences, Wuhan 430074, China;
3. Hubei Geological Survey Institute, Wuhan 430034, China
As one of the most important petroliferous lacustrine sag inthe Bohai Basin, many previous studies about Qikou sag has been done, including genetic types, sequence stratigraphy models of Paleogene slope break belts and lacustrine shale deposition (Yin et al., 2016; Huang et al., 2015, 2012; Li et al., 2012a, b; Liu and Zhang, 2011; Wang et al., 2011; Zhou et al., 2011; Qi and Yang, 2010; Lin, 2004); variable tectonic accommodation (Huang et al., 2015, 2012); recognition and depiction of special geologic bodies of Member 3 of Dongying Formation in littoral slope zone, and sequence thickness and its response to episodic tectonic evolution, as well as stratigraphic architecture and vertical evolution of various type structural slope breaks (Chen et al., 2014a, 2012, 2011); comparison of differences in sedimentary filling and its controlling factors in rift lacustrine basins of the Qikou and Nanpu sags (Wang et al., 2011); sand-accumulation and reservoir-controlling mechanisms of Paleogene slope-break system, and structural anatomy and dynamics of evolution of the Qikou sag studied the sedimentary evolution of overlapped sand bodies in terrestrial faulted lacustrine basin in Liaodong Bay of Bohai Basin (Yin et al., 2016; Zhou et al., 2012, 2011). This study focuses on the description and analysis of the special geological bodies of Ps1s (Shahejie Formation) during the stable rifting evolution stage by using the data sets of cores, well drilling, 3D seismic, inter-well correlation, and paleogeomorphology. The aims of this study are (1) recognizing and describing the special sandstone bodies under the stratigraphy framework; (2) documenting the internal architectures of sandstone bodies of Ps1s Formation, and (3) analyzing the sandstone developments during the evolution of the Paleogene, and discussing potential reservoirs.1 GEOLOGICAL SETTING 1.1 Paleogeography
The Huanghua depression, locating in the middle of Bohai Basin in Northeast China, is a typical reformed petroliferous depression developed since Mesozoic through Cenozoic age (Fig. 1) (Guo et al., 2013; Li et al., 2012a; Zhou et al., 2012). The Qikou sag locates in the middle part of the Huanghua depression, is a lacustrine rifting sag that has developed since Paleogene (Chen et al., 2012; Wang et al., 2009a).
In the Huanghua depression, the Qikou sag was encompassed by Cangxian uplift in the northwest, Chengning uplift in the southeast, Kongdian structural belt in the west, and Qikou coastwise belt in the east (Fig. 2). The exploration area is about 3 500 km2 (Chen et al., 2014b, 2012, 2011; Xiang et al., 2011; Xiang, 2009). The Qikou sag is composed of four sub-sags (see pink areas in Fig. 2) and four tectonic belts (see green areas in Fig. 2), which are Banqiao sub-sag, Qibei sub-sag, Qinan sub-sag, and Qikou main sag that distribute between the uplifts of Beidagang buried hill, Nandagang buried hill, Chengbei step fault zone, and Binhai tectonic belt, which means from north to south, the three sub-sags (Banqiao, Qibei, Qinan) are separated by three buried hills or uplifts (Beidagang buried hill, Nandagang buried hill, and Chengbei fault terrace) (Fig. 2).1.2 Tectonic Background
Huanghua depression has experienced five tectonic stages including fault subsidence period, extended fault subsidence period, sustained fault development period, depressed fault development period to the depression period (Qi and Yang, 2010). Huanghua depression shows superimposed tectonic features in strata stacking patterns in space and ordered features in time domain during evolution of regime processes due to its central location in the Bohai Basin (Li Z G et al., 2013; Huang et al., 2012; Li S Z et al., 2012a, b; Dong et al., 2010; Qi and Yang, 2010). The tectonic trending of Huanghua depression and associated Bohai Basin is northeast-southwest.
As the largest sag in the Huanghua depression, the Qikou sag is synchronous with the Huanghua depression, and has undergone the same tectonic history with the Huanghua depression. Thus, the sedimentation of Qikou sag represents episodic evolution features during Paleogene, which means that the Qikou sag has undergone the full series of initial fault depression stage, meridian stage, lacustrine basin migration stage, and decline stage of faulted basin in Palaeogene (Chen et al., 2012; Zhou et al., 2012; Xiang et al., 2011; Qi and Yang, 2010). This interphase distribution of tectonic units represents half-graben shapes that controlled by faults in the north, and gentle slopes in the south part of each sub-sag (Du et al., 2010; Wang et al., 2009a, b). (1) Banqiao sub-sag is controlled by Cangdong fault and Dazhangtuo fault, which formed steep slope on the north end and gentle slope on the south end; (2) Qibei sub-sag is controlled by Gangxi fault on the north, which formed fault controlled steep slope on the north and gentle slope on the south; (3) The Qinan sub-sag is bounded by steep Nandagang fault on the north and controlled by broom-like (plane view) Yangerzhuang fault on the south that formed Chengbei fault terrace. The cross section AA' (Fig. 2), which is across Qibei sub-sags from the west to east in Qikou sag, shows the stratigraphic frameworks of the Paleogene from Shahejie Formation to Guantao Formation.1.3 Stratigraphic Framework
The whole Paleogene strata are composed of 11 formations: Ps33, Ps23, Ps13, Ps2, Ps1x, Ps1z, Ps1s, Pd3, Pd2, Pd1s, and Pd1s(Fig. 3), which stretch across Middle Eocene to Late Oligocene in greenhouse climate condition (Zhang et al., 2016), as the lake levels are more dynamic and sensitive to climate changes and other boundary conditions with their much smaller volumes comparing to ocean systems (Bohacs et al., 2000). The episodic tectonic evolution of the Qikou sag during the Paleogene can be divided into three distinct rifting stages: an early rifting period (Ps33-Ps2); a stable rifting period (Ps1z-Ps1s); a rifting-depressed conversion period (Pd3-Pd1s) (Chen et al., 2012., 2011; Guo et al., 2011; Du et al., 2010).2 DATA SETS AND METHODOLOGY 2.1 Data Sets
The data sets that have been used in this study include 9 well logs, curves (SP, GR, and LLD), cores and 3D seismic data. The 3D prestack migration seismic survey data of the Qikou sag were acquired by PetroChina Dagang Oilfield Company.2.2 Methodology
The research approach of this study is the integration of well logs, cross sections, paleogeomorphology, 3D seismic amplitude attribute, and seismic profile, which provide factual information for detecting and identifying the comprehensive and elaborate architectures of sandstone bodies (Chen et al., 2014a, 2012, 2011; Jin et al., 2014, 2013; Bai et al., 2012).2.2.1 Well data and inter-well correlation
Two cross sections that across wells W1, H6, H1, H2, H3, J3, H4, H5, and W3 were interpreted with lateral continuities and sedimentary facies analysis based on well lithology, SP, GR, and LLD log curves (see the well logs of H3 and W1; core section of H2 in Fig. 4), sedimentary structures and facies, and depositional environments.2.2.2 Seismic data
The strata framework and boundaries were interpreted based on the seismic interpretations using reflection facies, and typical identifications of strata boundary, such as the onlap, downlap, toplap, truncation termination reflections, and wells with synthetic seismograms. The structure and fault system were constructed with considering of structural controls on strata morphology, fault distributions, and sedimentary structures based on horizon tracking and closing (Garciacaro et al., 2011; Catuneanu et al., 2009; Bohacs et al., 2000). Sedimentary structures in seismic profiles were used for correlation interpretation and source analysis in inter-well or no well regions (Song et al., 2014; Jiao et al., 1998).2.2.3 Paleo-morphology
The syndepositional morphology has been recovered according to back striping and decompaction correction, which exhibits the initial paleo-topography (Chen et al., 2014b; Li et al., 2004; Liu and Jiang, 1995; Dahlstrom, 1969).3 RECOGNITION OF THE SPECIAL GEOLOGIC SANDSTONE BODIES OF Ps1s FORMATION IN QIBEI SUB-SAG 3.1 Description
The well log of H3 shows interbedded thin sandstones and mudstones alternations, with a relative thick sandstone layer at the level of 3650-3625 m. The well log of W1 illustrates a mud-stone background. The core section of H2 represents fine sand-stone interval with reverse graded bedding features (upward coarsening) from 3 637.28 m to 3 624.31 m (Fig. 4).
The statistics data of impedance and gamma ray curves from the well logs are shown in Fig. 5. The good relevance cross plot relationship shows the distributions of mudstones in higher GR and lower IMP area, whereas fine-grained sandstones and medium grained sandstones in higher IMP but low GR area, which shows good response relationship between the impedance, gamma ray and lithology in the well logging. The impedance range of sandstone is about 8 000-10 500 Ohm.
The well-log cross-sections show the internal architecture of the sandstone bodies (Figs. 6, 7). Cross section EE' is across the fan front with the sandstone bodies are present in the wells H1, H2, and H3 (see well log and core of H2 and H3 in Fig. 4) in the lower Ps1s Formation, pinching out towards Well H6 in southwest (Fig. 6).
Compare to the proximal CC' section, the sandstone bodies shown in EE' indicate a distal location with a narrow distributed arrange. Well cross section FF' is across near the east boundary of the sandstone bodies bodies along source to sink trend, which exhibits sandstone bodies in wells H4, H5, and H2, and pinchout at the updip point towards Well W3 to the south. The main sandstone depocenters distribute at H5 and H2 (Fig. 4c). These sandstone bodies also thin towards wells J3 to the north (Fig. 7). The dimension of the sandstone bodies is about 10 km width along SW-NE (CC' and EE') and 7-8 km long along NW-SE (DD' and FF'), with the thickness about 50-70 m. The sandstone bodies have changed from distributing in the center part of sub-sag during the early Ps1s period in the lower Ps1s Formation, to the relative separated distribution during the late Ps1s period in the upper Ps1s Formation (Figs. 6, 7). This northwest-southeast orientated sandstone bodies can be recognized from the seismic profile CC' and DD' (sandstone bodies are highlighted in the light blue color in Fig. 8). The seismic reflections of the lower Ps1s Formation are composed of overlapped lineups pinching out at updip or downdip points. Each of the formations (Ps1s, Ps1z, Ps1x) has approximate 200 milliseconds vertical time duration.
The average Vp velocity of stratified sediments and the basement rocks of Paleogene in Bohai Bay Basin is 2.9 km/s with the Vs of 1.7 km/s (Zhao and Zheng, 2005). The sandstone bodies developed on the hanging wall of the growth fault are influenced by the tectonic movements of Gangdong fault, although in general the fault activity rate is much slower than the sediment rate (Li S et al., 2012).
The paleo-morphology clearly shows that the uplift of the Beidagang buried hill is the source area and the sub-sag of Qibei (Fig. 9) is sedimentation zone (see the green survey area in Fig. 2). As one of the main sub-sag during the Ps1s period, the Qibei sub-sag received deposits from the northwestern Beidagang buried hill through the relative steep slope. Controlled by the growth fault that sitting right at the boundary of Beidagang buried hill and Qibei sub-sag, a subaqueous fan complex developed along the steep slope down to the center of the sub-sag. The geomorphology of the sub-sag provides profitable conditions to form potential accommodation for depocenters in lower terrain center and updip traps towards surrounding areas with the higher terrain.
The seismic average absolute amplitude map illustrates the plane distributions of this fan in the lower Ps1s Formation, as the warm color (red and orange) shows the high amplitudes (Fig. 10a). The plane area of the Ps1s Formation subaqueous fan is about 8×8 km2. The thickness of the sandstone body complex is up to 70 m (Fig. 10b), which disperse into the south, southwest, and east along the fault trends. The sedimentary facies model is shown in Fig. 9c, which indicates two sources that controlled by Gangxi (fault transfer zone) and Gangdong (fault terrace zone) fault transfer systems from northern and north-western directions. The north-western source has been transported from Gangxi fault down to Gangdong fault, as the north-eastern source has been transported along the Gangdong fault, which merged together at the downthrown side of Gangdong fault.3.2 Interpretation
According to the core, well log, cross plot of impedance and gamma ray, cross section of well logs, seismic profiles, and paleo-geomorphology analysis, along with the seismic attribute, sandstone thickness, and sedimentary patterns, a subaqueous fan complex has been recognized in the Ps1s Formation. The core and well log data of H2, H3, and W1 represent the sedimentary facies from proximal to distal, which are fan front, fan boundary, and semi-deep lake environments respectively, with decreasing sandstone content (Fig. 4). The good response relationship of the cross plot of impedance and Gamma Ray indicates the fine and medium grained sandstone cluster in the higher IMP but low GR area (Fig. 5). The well-log cross-sections along both SW-NE and NW-SE directions demonstrate the architecture of sandbodies in the Ps1s Formation, which show a lenticular shape in cross section EE' with southwest edge and east edge pinching out (Fig. 6). And in the cross section FF', the sandbodies lie on the downthrown side of Gangdong fault, and pinch out towards the distal part of the subaqueous fan (Fig. 7). The seismic profiles responded to the cross section in both directions (profile CC' and DD') with bidirectional wedge out (onlap on west side and downlap on east side) in profile CC' and unidirectional downlap towards southeast in profile DD' (Fig. 8).
The paleo-morphology shows clearly with the uplift and sub-sag pattern of Beidagang buried hill and Qibei sub-sag, which provides both sedimentation sources from the raised uplift and accommodation space in the low-lying land of sub-sag. This geomorphology controlled the distribution of the subaqueous fan sedimentary system (Fig. 9). In the plane view from the seismic attribute map and sandstone thickness map, the distribution of the subaqueous fan developed from northwest fault terrace towards southeast direction (Fig. 10a). The lobe shape has revealed in the sandstone thickness map (Fig. 10b). There are two sources with two types of fault transfer patterns, one is from the fault transfer zone of Gangxi fault, and another is from the fault terrace zone of Gangdong fault (Fig. 10c).4 DISCUSSIONS 4.1 Ps1s Formation in the Process of Regime Evolution
Ps1s Formation is the transitional period that the Qikou sag evolved from rifting to depression, which is also the strata boundary that sedimentary process changed from transgressive to regressive. The subsidence of Ps1s became slow after rapid decreasing of tectonic movements during Ps3, Ps2, Ps1 (Li et al., 2012a; Qi and Yang, 2010). The subsidence centers of the Qikou sag have migrated from dispersing in sub-sags towards gathering in the north-eastern area of Qikou. This trending is also shown in the paleo-morphology map of Ps1s, as it is an important epoch boundary between Eocene and Oligocene.4.2 Reservoirs
The study of the sandstone bodies helps to address their relationships with hydrocarbon play elements-source, reservoir and seal (Li S L et al., 2015; Bohacs et al., 2000). The special geological sandstone bodies reveal different potential subtle traps and reservoir types in the areas with available well data, which including the lithologic lens traps (wells H1 and H2), lithologic updip pinchout traps (Well H2), and structural-lithologic composite traps (Well J3) (Mao et al., 2005). The uplift of the Beidagang buried hill provides sediments for the Qibei sub-sag. Ps3 and Ps2 strata, which was deeply buried ( > 2 600 m), worked as the source rock for hydrocarbon expulsion (Guo et al., 2011). And the widely developed mudstone overlying on the top of Ps1s is the ideal cap rock in the reservoir-seal combination (Du et al., 2010). The continuously developed fault systems and the regional unconformity boundary under Ps1x provide migration pathway for oil and gas transporting from underneath source rock to potential subtle traps (Bai et al., 2011; Guo et al., 2011; Gabrielsen et al., 1995). All of these provide favorable conditions for potential reservoirs in the geological sandstone bodies of the Ps1s Formation.4.3 Other Control Factors
The strata of lacustrine basin are the results of complex control factors. The architecture and strata patterns are the results of the balance between accommodation and sedimentation (Bohacs et al., 2000). Other control factors that can be considered in further work include the (1) high-frequency fluctuations of lake levels controlled by climate during the greenhouse interval; (2) Milankovitch-scale climatic cycles, which produced lake-level cycles, caused by variations of the Earth's axis and orbit (Li S et al., 2015; Bohacs et al., 2000; Olsen, 1990); (3) rate of change of accommodation (Muto and Steel, 1997).5 CONCLUSIONS
(1) The sediments from the northwest provenance of Beidagang uplift have been transported into Qibei sub-sag, which are the main deposit input during the Ps1s period. The dimensions of the special sandstone bodies is about 8×8 km2 with 50-70 m thickness.
(2) The special geological bodies of Ps1s Formation developed in a transitional period that depositional processes have changed from transgression to regression, and as a strata boundary that the Qikou sag switched from relative high tectonic activities to stable subsidence period.
(3) Special geological bodies are efficient subtle traps for lithology reservoirs and updip pinchout reservoirs, and also, provide scientific basis and geological models for the predictions of potential reservoirs, and also for frontier basins with little available data.
This paper benefited from data provided by the PetroChina Dagang Oilfield Company. The authors would like to express their appreciation for the help provided by colleagues of Petro China Dagang Oilfield Company. Thanks to the China University of Geosciences (Wuhan) for administrative support, and thanks to the National Key Projects of China (Nos. 2011ZX05009-002, 2016ZX05006006-002) for financial support. The final publication is available at Springer via https://doi.org/10.1007/s12583-016-0937-4
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