Cretaceous formation in the western Tabei Uplift is significant petroleum pay zone in the Tarim Basin, NW China. For the complex relationship between tectonic movement and depositional environment, sequence stratigraphic filling model of the whole Cretaceous formation has not been established yet. Using well logs and seismic data in this article, sequence stratigraphic evolution process and filling model of the Cretaceous are interpreted involving paleo-uplift, sedimentary and water-level. As a first-order sequence, the Cretaceous is subdivided into three second-order sequences, Shushanhe, Baxigai and Bashijiqike Formation. Shushanhe Formation is subdivided into three third-order sequences, KSQ1, KSQ2 and KSQ3 upwards; Baxigai Formation is considered as one single third-order sequence, KSQ4; Bashijiqike Formation is subdivided into three third-order sequences, KSQ5, KSQ6 and KSQ7 upwards. In the early KSQ1, paleo-uplift was aquatic. From KSQ1 to KSQ3, water-level raised and paleo-uplift moved upwards gradually; and water-level raising was faster than paleo-uplift ascending. The accommodations of these three sequences increased. Lacustrine facies mudstones and siltstones were onlap filling sequences towards Xiqiu Paleo-uplift. In the end of KSQ3, the whole region was subaqueous. The water-level and paleo-uplift in KSQ4 were easy steady, and the accommodation in KSQ4 was sustained. Delta depositional system began to prograde from depocenter to distal slope of paleo-uplift. In the high of Xiqiu Paleo-uplift, dominated sediments were fine grains for the sedimentary dynamic force. From KSQ5 to KSQ7 in the Bashijiqike Formation, water turned regression, paleo-uplift raised slowly and the accommodation reduced continually. Because of the progradation deltaic sandstones, downlap sequences were developed towards the Xiqiu Paleo-uplift. Exposed in the Early Paleogene, KSQ7 and part KSQ6 were truncated to form top unconformable boundaries. Established sequence filling model of the Cretaceous in the western Tabei Uplift provides important insights on the stratigraphic characteristics and in turn on the exploration potential of petroleum systems.
Sequence stratigraphy analyzes the sedimentary response to changes in relative sea- (lake-) level, and the depositional trends that emerge from the interplay of accommodation (space available for sediments to fill) and sedimentation . Sequence stratigraphy has tremendous potential to interpret the basin’s geological record and changes, and to improve the petroleum and mineral exploration and development [2,3]. For these reasons, sequence stratigraphy is currently one of the most active areas of research in oilfields. The Tarim Basin, the largest depositional basin in China, is in southern Xinjiang Uygur Autonomous Region. In northern Tarim Basin, Kela-2 and Keshen-2 large-scale gasfields were found in the Cretaceous of Kuqa Depression, on the north of Tabei Uplift [4–6]. The foreground of petroleum exploration in the Cretaceous of the western Tabei Uplift was proved be excellent by several lithologic hydrocarbon reservoirs [7–10]. Although, some stratigraphic research studies have been carried out in the partial Cretaceous in recent years, these works mainly focused on depositional evolvement [11–14]. Sequence stratigraphic filling process and model of the whole Cretaceous are not reported completely yet.
The change of relative lake-level is the cooperative result of tectonic action, sedimentary supplementation and climate. Furthermore, change of relative lake-level controls sequence development. Based on well and seismic data, classic sequence stratigraphy theory is used to identify and compare the sequence of the Cretaceous in the western Tabei Uplift vertically and horizontally. In the sequence stratigraphic framework, sequence evolution is analyzed from three factors of paleo-uplift, sedimentary and water-level. Sequence stratigraphic filling model of the Cretaceous in the western Tabei Uplift is finally established in this article.
2 Geological setting
Tarim Basin, one of the most significant petroleum basins in northwestern China, contains many gas-producing depressions, such as Kuqa Depression and North Depression in the north (Figure 1a and b). Tabei Uplift is just a first-order tectonic unit between these two depressions. Tabei Uplift consists of several second-order salient and sags. The study area of this article is in the west of Tabei Uplift, including part Yingmaili Salient and Luntai Salient (Figure 1c). Before Cretaceous Period, geography is south-toward slope, high in north and low in south . In the north of the region, a large-scale Xiqiu Paleo-uplift and a small-scale Xinhe Paleo-uplift were developed (Figure 1d) [12,16]. It should be noted that the Cretaceous was supplied by lacustrine delta depositional system from the southeast and not from the north [10,11,12,13]. Both paleo-uplifts would affect stratigraphic characteristics.
The Cretaceous overlies on the Jurassic, Triassic, Permian, Silurian or Ordovician, and is covered by Paleogene uncomfortably. Based on chronostratigraphy, the Cretaceous within thickness of 600–1,200 m is divided into three formations upwards, Shushanhe Formation (K1ss), Baxigai Formation (K1bx) and Bashijiqike Formation (K1bs). In Shushanhe Formation, 0–600 m thickness, reddish-brown clays and thin gray-green silts were alternately deposited in lacustrine facies [11,12,13]. In Bashijiqike Formation, 100–600 m thickness, dozens of meters-thickness reddish-brown sands are dominated sediments in delta facies, imbedded thin clays and silts . Thickness of Baxigai Formation is homogeneous in 40–60 m, and the sediments are transition interval of lower sands and upper clays (Figure 1e).
3 Database and methodologies
The involved data in this article are mainly well logs and seismic data. Drill cuttings in the mud log are important understanding on the lithology of formation. Conventional wireline logs represent geophysical recordings of various rock properties in boreholes, and can be used for geological interpretations. Gamma ray (GR; API) is used in single well and multiple wells in this article. In the Cretaceous formation, sands are usually in low GR while muds are in high GR (Figure 1e). The most study area is covered by 3D seismic. Seismic data provide the fundamental means for the preliminary evaluation of a basin fill in the subsurface, in terms of overall structure, stratigraphic architecture and fluid content. In order to transform depth domain to time domain in seismic, well-to-seismic has been calibrated in key wells. Then, depth and amplitude intensity of seismic reflection interfaces were traced in different sections to recognize the sequence boundary.
Proposed by Vail et al. in the 1970s and improved by Exxon research group, sequence is defined as a set of relatively integrated, genetically related stratigraphic units whose top and bottom are bounded by unconformities or correlating interfaces with these unconformities [17–20]. Based on comprehensive understanding of previous research, drilling, logging and seismic data, sequence classification scheme of the Cretaceous is prepared. Cretaceous is as a first-order sequence which is subdivided into three second-order sequences corresponding to Shushanhe, Baxigai and Bashijiqike Formation. Shushanhe Formation is subdivided into three third-order sequences, named KSQ1, KSQ2 and KSQ3 upwards [11–13]; Baxigai Formation is considered as one single sequence, KSQ4 [11–13]; Bashijiqike Formation is subdivided into three sequences, termed KSQ5, KSQ6 and KSQ7 upwards [11,14]. Totally seven sequences are recognized in the whole Cretaceous (Figure 2). In the classic sequence model advocated by Vail, each I-type third-order sequence can be subdivided into Lowstand Systems Tract (LST), Transgressive Systems Tract (TST) and Highstand Systems Tract (HST).
In the Cretaceous in the western Tabei Uplift, dissecting sequence stratigraphic filling characteristics requires knowledge of the primary depositional and stratigraphic features. This analysis is carried out in four stages in the current study. Dividing sequences in single well using logging and seismic data is the first stage. Representative sequence boundaries are described vertically. In the second stage, sequences in different locations are compared. Sequence boundaries in wells and seismic sections are traced horizontally embracing paleo-uplift region. In the third stage, the sequences and facies are superimposed to combine with the depositional characteristics. In the fourth stage, sequence evolution process is analyzed upwards. Finally, sequence stratigraphic filling model is formed involving factors from paleo-uplift, sedimentary and water-level.
4.1 Vertical sequence
First, combined with sequence stratigraphy and well-to-seismic calibration, the vertical sequences are identified in wells. The sequences in well YM7 are shown in Figure 2 for example. In the well-to-seismic calibration, all sequence basal boundaries match wave troughs in seismic profile. In this well, basal boundary of KSQ1 is the interface of overlying mud in the Cretaceous and underlying carbonate in the Ordovician. Lower muds and silts developed in LST and TST; upper silts deposited in HST. Basal boundary of KSQ2 is the interface of GR changing from high to low. Lower thick silts accumulated in LST, middle muds and silts interbeds developed in TST and upper thick silts deposited in HST. Basal boundary of KSQ3 is the interface between thick silts in HST of SQ2 and mud-silts in LST in SQ3. Basal boundary of KSQ4 is the base of thick sands deposited in LST; muds and silts developed in TST and HST. In addition, sandstones in LST of KSQ4 are the main petroleum reservoir. Thick sands developed in KSQ5, KSQ6 and KSQ7 (Figure 2). These basal boundaries correspond to high GR interfaces. Using this method, vertical sequences are divided in wells one by one.
4.2 Horizontal sequence
Second, based on vertical sequence boundaries in wells and seismic sections in different regions, sequence superposition could be laterally correlated. The base and top boundaries of the Cretaceous (first-order sequence boundaries) are large-scale unconformable interfaces in the whole basin [16,21]. Seismic reflection of the basal boundaries of the Cretaceous is in middle amplitude within poor continuity; while, seismic reflection of the top boundaries of the Cretaceous is in strong amplitude within well continuity. The base and top boundaries of SQ4 (second-order sequence boundaries) are parallel unconformity or local angle disconformity in this study area [11,22]. Both these two interfaces are strong amplitude within well continuity, which is clear in seismic sections (Figures 3 and 4). However, other boundaries of third-order sequences are weak amplitude and poor continuity in seismic, which must be combined with well logs. Figure 3 displays the horizontal sequence development from depocenter to Xiqiu Paleo-uplift in Section-1.
According to seismic and logs, it is obvious that the thickness of the Cretaceous thinned towards the paleo-uplift. Specially, KSQ1, KSQ2 and KSQ3 were pinch-out at the slope of paleo-uplift gradually. There is no KSQ1 and KSQ2 developed in the uplift high, and KSQ3 was just deposited locally. They were onlap filling sequences, and corresponding onlap points were on the slope of paleo-uplift. This would be explained in the next section.
KSQ4 in the Baxigai Formation covered all regions, including in the uplift high with little change in thickness. Although the Bashijiqike Formation was in the same thickness variation trend with Shushanhe Formation (thinning towards paleo-uplift), both sequence filling models were totally different. Within KSQ5 and KSQ6, both LST were downlap towards the paleo-uplift, which would be explained in the next section. Due to the structural inversion in the Early Paleogene, KSQ7 and HST of KSQ6 were gradually truncated by the base of Paleogene towards the paleo-uplift. Specially, KSQ7 may all corrode out in the uplift high (Figure 3).
Sequences across Xiqiu and Xinhe Paleo-uplift are correlated in Section-2. There are similarities in the cross-section of both paleo-uplifts. KSQ1 and KSQ2 were onlap sequences towards both the paleo-uplifts. KSQ1 and part KSQ2 were not developed in the high of Xinhe Paleo-uplift. It means that the Cretaceous formation was deposited from KSQ2. Other upper sequences were affected by few Xinhe Paleo-uplift (Figure 4).
5.1 Sequences and facies
Third, according to the lithology and thickness of sediments in different sequences in the regional depositional setting, sequences and facies are simply discussed. The local sedimentary characteristics in the Tabei Uplift were controlled by overall depositional system in the Tarim Basin. In the Cretaceous at this research region, sandy sediments were supplied by lacustrine delta from southeast to northwest [10–13,21]. As a matter of fact, with affluence and uprising of the water, the paleo-uplift was just an obstacle which would be covered by sediments finally. This special depositional characteristic easily results in misunderstanding that the water and sediments are moved from lowland to highland.
In the Shushanhe Formation, lithology is thick fine grains, mudstones and siltstones, nearly no sandstones. It suggests that the facies are mainly lacustrine with gradually ascending water-level. Therefore, for the lack of sandy sediments and high water-level, the KSQ1, KSQ2 and KSQ3 were distal onlap lacustrine muds and silts. The KSQ4 was dominantly composed by lower sandstones and upper mudstones. It means that sands transported by delta depositional system began to accumulate from depocenter to slope of paleo-uplift. Limited by distance and sedimentary dynamic force in the high of Xiqiu Paleo-uplift, the KSQ4 was just deposited by thick mudstones and siltstones. The KSQ4 distributed widely and its thickness is uniform, about 40–60 m, in the whole region. In the Bashijiqike Formation, the sediments are thick sandstones and thin mudstones. It indicates that the water-level turned down and deltaic depositional system prograded far. Therefore, with the abundant sediment supplement and descending water-level, there were downlap, not onlap, that existed in the KSQ5 and KSQ6 (Figure 5).
5.2 Sequence evolution
Fourth, based on the vertical and horizontal sequence characteristics, sequence evolution process of the Cretaceous is dissected from paleo-uplift activity, depositional system and water-level change (Figure 6). In the early of the Cretaceous, the paleo-uplift was aquatic [9,12,15,16]. From KSQ1 to KSQ3 in the Shushanhe depositional stage, water encroached into this research region, and the water-level raised gradually. At the same time, the paleo-uplift moved upwards as well. However, water-level raising was faster than paleo-uplift ascending, which resulted in the rise of relative lake-level. It means the increase of accommodation. Without sandy grains supplement, thick mudstones and thin siltstones from lacustrine facies gradually developed in the onlap filling sequences on the slope of paleo-uplift. In KSQ2, Xinhe Paleo-uplift was submerged; in the end of KSQ3, water completely covered the whole region, and the Xiqiu Paleo-uplift was entirely subaqueous (Figure 6a–c).
The water-level and paleo-uplift in KSQ4 would be weak in activities. Therefore, the accommodation in KSQ4 was sustained. Far away from sedimentary provenance, dominated sediments of KSQ4 change from sands to fine grains towards Xiqiu Paleo-uplift. And in the high of Xiqiu Paleo-uplift, dominated sediments of KSQ4 is mudstones and siltstones (Figure 6d).
Even though the variation in thickness of Bashijiqike Formation is similar with Shushanhe Formation, their sequence evolutions were totally different. From KSQ5, water turned regression, the paleo-uplift raised slowly and relative lake-level went down [9,12,15,16]. The accommodation of KSQ5, KSQ6 and KSQ7 reduced continually. Meanwhile, deltaic sandstones prograded to the paleo-uplift gradually, leading in the downlap sequences towards the Xiqiu Paleo-uplift (Figure 6e–g). This is readily-easily to be misjudged with onlap in KSQ1, KSQ2 and KSQ3. Exposed in the Early Paleogene, KSQ7 and part KSQ6 were truncated to form top unconformable boundaries (Figure 7). Sequence stratigraphic filling model of the Cretaceous in the western Tabei Uplift, controlled by paleo-uplift, sedimentary and water-level, is established finally.
Sequence stratigraphic filling model of the Cretaceous in the western Tabei Uplift is established using wells and seismic data. Seven third-order sequences are recognized in the whole Cretaceous upwards: lower Shushanhe Formation is subdivided into KSQ1, KSQ2 and KSQ3; middle Baxigai Formation is as KSQ4; upper Bashijiqike Formation is subdivided into KSQ5, KSQ6 and KSQ7.
In the early KSQ1, paleo-uplift was aquatic. From KSQ1 to KSQ3, water-level raised gradually and paleo-uplift moved upwards. However, water-level raising was faster than paleo-uplift ascending, which resulted in the rise of relative lake-level and the increase of accommodation. Without sufficient sandy sediments, lacustrine facies mudstones and siltstones were onlap filling sequences towards Xiqiu Paleo-uplift. In the KSQ2, Xinhe Paleo-uplift was subaqueous; and in the end of KSQ3, the whole region was subaqueous.
The water-level and paleo-uplift in KSQ4 were easy steady, and the accommodation in KSQ4 was sustained. Delta depositional system began to prograde from depocenter to the slope of paleo-uplift. In the high of Xiqiu Paleo-uplift, dominated sediments were fine grains for the sedimentary dynamic force. KSQ4 distributed widely in the whole region. From KSQ5 to KSQ7, water turned regression, paleo-uplift raised slowly and the accommodation reduced continually. For the progradation deltaic sandstones, downlap sequences were developed towards the Xiqiu Paleo-uplift. Exposed in the Early Paleogene, KSQ7 and part KSQ6 were truncated to form top unconformable boundaries.
The authors are thankful the anonymous reviewers for their constructive reviews on the manuscript, and the editors for carefully revising the manuscript. This research is financially supported by the National Natural Science Foundation of China (No. 42202113) and Scientific Research Project of Hubei Provincial Department of Education (No. Q20211302).
Conflict of interest: Authors state no conflict of interest in this article.
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