Yanchang Formation of Upper Triassic in northern Ordos Basin is a lake-transgression-lake-regression cycle, and the sedimentary evolution sequence is river → shallow water → semi-deep lake → delta → river → swamp facies, which can be divided into ten oil-bearing formations from top to bottom, including Chang 6 oil-bearing formation, in which Zhidan Delta in northern Shaanxi is bird-footed (Figure 8-65438). The reservoir skeleton sand body is an underwater distributary channel sand body with good physical properties, thin interbedded sand and mudstone and a labyrinth structure with strong heterogeneity (Du et al., 1999). According to the cyclicity of formation, Chang 6 oil formation can be subdivided into three oil layers and six small layers (Table 8- 1). Based on the sedimentary dynamics analysis of base-level cycle structure and superposition style (Zheng et al., 2000) and the multi-level sub-cycle sequence division scheme (Zheng et al., 200 1), this paper discusses the high-resolution sequence development characteristics and isochronous correlation technology of the lake delta system, as well as the research significance of sequence stratigraphic framework at different time scales in various stages of oil and gas exploration and development.

Second, the characteristics of high-resolution sequence stratigraphy

Types and genetic characteristics of (1) base-level cycle interface

Interface identification is the basis of dividing sequence, making regional isochronous stratigraphic correlation and establishing chronostratigraphic model. Among the six kinds of base-level cycle interfaces commonly developed in continental oil-bearing basins (Zheng et al., 200 1), Yanchang Formation can identify five kinds of interfaces, namely, type II and III controlled by structural factors and type IV, V and VI controlled by astronomical cycle climate fluctuation factors. In addition to the type II interface marked by regional structural unconformity at the bottom and top of Yanchang Formation, the interface of Chang 6 oil-bearing formation mainly consists of ten regional isochronous marker layers of tuff or tuff mudstone marked by K0-K9, and Chang 6 oil-bearing formation has three tuff marker layers, namely K2, K3 and K4 (Table 8- 1). Among them, K2 is located at the bottom of Chang 6 oil-bearing formation, which is the boundary sign with Chang 7 oil-bearing formation, and K4 is located at the top of Chang 6 oil-bearing formation, which is the boundary sign with Chang 4 oil-bearing formation. The identification of the above sequence boundary and three marker beds provides an important basis for high-resolution sequence division, isochronous correlation and establishment of chronostratigraphic framework of Chang 6 oil-bearing formation.

Fig. 8- 1 Paleogeographic Schematic Diagram of Chang 6 Period in Late Triassic in Ordos Basin

(2) Division and characteristics of cyclic sequences of different orders.

According to the development of various sequence interfaces and the response characteristics of stratigraphic processes recorded by different levels of datum level changes, Chang 6 oil-bearing formation can be divided into 1 long-term, 4 medium-term, 22-24 short-term and more ultra-short-term cyclic sequences (Figure 8-3). The sedimentary sequences, structural types and superimposed styles of each cyclic sequence are briefly described as follows.

Table 8- 1 Summary of Oil Layer, Sub-layer Division and Lithologic Combination Characteristics of Chang 6 Oil Group

1. ultrashort periodic sequence

This kind of sequence is the smallest genetic stratigraphic unit divided according to the actual data such as core or logging, and the sequence thickness varies from several meters to less than 1 meter, with the VI interface as the sequence boundary. There are various genetic types of the interface, which are mainly manifested as small-scale scouring surface and inflow scouring surface formed in the process of rapid change of accommodation space and sediment supply (A/S) and sedimentary dynamic conditions, or under-compensated sedimentary discontinuity and integration surface formed under stable conditions. Defined sequences often have different cyclic structures and superimposed styles (Figure 8-4), which can be subdivided into three basic types and seven subtypes. The characteristics of each type and subtype are as follows.

Fig. 8-2 Identification marks and production characteristics of base-level cycle interfaces in oil layers (according to ZJ39 well Chang 6 12 oil layers)

Figure 8-3 Comprehensive Histogram of Sedimentary Facies and High Resolution Sequence Stratigraphy of Chang-6 Oil-bearing Formation

Figure 8-4 Sequence Structure of Several Ultra-short-term and Short-term Cycles in Chang 6 Oil Formation

The asymmetric upwelling "deep" is the most common type in the sedimentary area of underwater distributary channel, and only the sedimentary records of the upper half cycle of the datum level are kept in the sequence, and the lower half cycle is in the state of erosion and erosion, so it has the asymmetric upwelling "deep" structure. According to the sedimentary facies combination characteristics of sequences, they can be divided into two subcategories: low accommodation space and high accommodation space.

(1) Asymmetrical subtype in which the low accommodation space becomes "deep" upward: it is composed of bottom scouring and single upward tapering distributary channel thick massive sandstone (A 1 in Figure 8-4), and it is composed of rapid shallow water flow, slow lake rapid propulsion and A/S? Under the sedimentary condition of 1 (Zheng et al., 2000), it mainly appears in the upper reaches of the underwater plain where the underwater distributary channels overlap and the lateral migration is very active;

(2) Asymmetrical subtype in which the high-accommodation space becomes "deep" upwards: it is composed of bottom scouring → thick massive sandstone of underwater distributary channel → thin siltstone of underwater natural dike → distributary bay mudstone (A2 in Figure 8-4), which is formed in deep water, but its flow speed is fast, and it enters the lake slowly and retreats quickly, with A/S < 1.

Asymmetric type, shallowing upward. This type is common in Yuanshaba-Hekouba sedimentary area. There is only one semi-cycle of descending datum level in the sequence, and the ascending semi-cycle is characterized by water entering the scouring surface or undercompensated sedimentary discontinuity, with an asymmetric structure of shallowing upward. According to the sedimentary facies combination and interface characteristics of the sequence, it can be divided into two subtypes: low accommodation space and high accommodation space.

(1) Asymmetrical subtype in which the low accommodation space becomes shallower upward: it is composed of a single thick layered sandstone at the inlet scour surface and estuary dam (B 1 in Figure 8-4), which is formed in shallow water, fast entering the lake, slow retreating the lake and A/S < 1, and is mainly produced in sediments with high sedimentation rate.

(2) Asymmetrical subtype in which the high accommodation space becomes shallower upward: the sequence that becomes shallower upward and slightly thicker is composed of undercompensated sedimentary discontinuity → thin interbedded mud of estuary dam, siltstone → thin sandstone with mudstone (Figure 8-4 B2), which is formed under the sedimentary conditions of rapid lake invasion and slow lake regression, with A/S > 1, and mainly appears in distant estuaries with low sedimentation rate.

Symmetrical type of "deep" and then shallow upward change. This type is developed in the estuary and its adjacent sides where the underwater distributary channel and estuary dam alternate, and has a symmetrical structure of "deep" and then shallow upward change, because the semi-cyclic sedimentary records of datum level fluctuation are preserved in the sequence. However, the symmetry change, sedimentary facies combination and interface characteristics of the estuary and its adjacent sides are also different, which can be roughly divided into three basic subtypes.

(1) The thickness of the ascending half cycle is greater than that of the descending half cycle (C 1 in Figure 8-4), and it is composed of scouring surface → thin siltstone of underwater natural dike → mudstone of distributary bay → thin sandstone in crevasse fan (or crevasse channel) from bottom to top, which is common in the inner side of estuary.

(2) The rising half cycle and the falling half cycle are completely symmetrical (C2 in Figure 8-4), and the thickness is almost equal. The sequence consists of weak scour surface (or abrupt change surface of sandstone and mudstone) → layered sandstone of underwater distributary channel → thin siltstone in underwater natural dike → mudstone of distributary bay or pre-delta → thick sandstone of estuary dam, which is common in estuaries.

(3) The thickness of the descending half cycle is greater than that of the ascending half cycle (C3 in Figure 8-4), which is found in the estuaries of floodplains and underwater plains. The floodplain in the underwater plain is composed of related integration surface → thin siltstone in the underwater floodplain → distributary mudstone → crevasse fan (or crevasse channel) and medium-thick siltstone, and it is composed of related integration surface (or water inflow scouring surface) at the estuary.

It should be pointed out that these three subtypes are all formed under the sedimentary conditions of water depth, slow flow velocity, slow lake advance and retreat, and A/S≥ 1 in medium and high accommodation space.

Ultra-short-term cycle sequence distribution model Based on the statistics of ultra-short-term cycle sequence distribution with different structures in different sedimentary facies belts in the delta, an ultra-short-term cycle sequence distribution model is proposed (Figure 8-5). From the variation law of cyclic structure along the delta growth axis in this model, it can be reflected that when the datum level is in a low-amplitude rising period, the deposition mainly develops in the underwater plain deposition area, and the deposition intensity of underwater distributary channel sand body weakens from upstream to downstream and estuary, and the overflow deposition increases. With the rise of lake water level and the migration of effective accommodation space to the land direction, the sediment supply in the estuary direction decreases, and the estuary dam and lake sedimentary area gradually enter an under-compensated sedimentary state; When the datum level is in a period of low decline, with the decline of lake water level and the migration of effective accommodation space to the basin, the underwater plain sedimentary area enters a state of erosion, and the erosion intensity and amplitude decrease from upstream to estuary, and the re-transported sediments produced by erosion gradually increase and are transported in the same direction as the sediments from the provenance, which leads to the increase of sediment supply and the increase of sedimentary intensity in the estuary dam and lake sedimentary area, with the estuary dam sedimentary area near the estuary being the strongest.

Figure 8-5 Distribution of Ultra-short-term and Short-term Cyclic Structure of Chang 6 Oil Formation and Correlation Model of Adjacent Sequences

2. Short-term cyclic sequence

This sequence is several meters to nearly ten meters thick, and the V-type interface is the sequence interface. Generally, it is formed by superposition of several ultrashort cycle sequences with similar structure and lithologic combination, and some of them are equivalent to a single ultrashort cycle sequence. The structure and distribution of the sequence are basically consistent with the ultra-short-term cyclic sequence (Figure 8-4 and Figure 8-5), and can be subdivided into three types: upward "deep" asymmetric sequence, upward-shallow asymmetric sequence and symmetric sequence, and seven subtypes, such as low or high accommodation space, symmetry or incomplete symmetry, indicating that these sequences have similar sedimentary dynamics formation conditions with ultra-short-term cyclic sequences.

3. Mid-cycle sequence

This sequence is tens of meters thick, with class ⅳ interface as the sequence interface. Chang 6 oil-bearing formation marked by K2 and K4 can be divided into four middle cycle sequences, which are named MSC 1, MSC2, MSC3 and MSC4 respectively from bottom to top (Figure 8-3), and the sequence structures are all half-cycle with incomplete symmetry. In the sedimentary evolution sequence, a middle cycle sequence represents a large-scale lake invasion-lake regression process, corresponding to the growth cycle of a delta sedimentary system from strong active progradation to accretion retrogression and then back to forced progradation (Figure 8-6). The interface of each mid-cycle appears between the estuary bar sand body in the early stage of forced progradation and the underwater distributary channel sand body in the later stage of forced progradation, resulting in a large undercut. Therefore, although the main body of the mid-cycle sequence is roughly equivalent to each small layer or oil layer, the interface position is not consistent with the traditional position where the bottom boundary of sandstone or the top boundary of mudstone is a layered boundary (Figure 8-3). On the vertical section, the ascending semi-cycle of each mid-cycle is mainly composed of a series of asymmetric short-cycle sequences of A 1 and A2, which represent the gradually upward "deep" water body, and partially asymmetric C 1 or C2 of the ascending semi-cycle appears in the upper part. The descending semi-cycle is mainly composed of a series of asymmetric short cycles B 1 and B2, which represent the upward shallowing of water bodies. The descending semi-cycle mainly includes nearly symmetric C2 and incompletely symmetric C3. The mid-term flood surface is composed of mudstone top surface developed in the transition zone where the base level rises and falls (Figure 8-3). On the longitudinal section parallel to the advancing direction of the delta, the sequence structure of middle cycle in different sedimentary facies belts is relatively stable, and generally, it is dominated by ascending semi-cycle type and symmetrical type. The structural change process and distribution pattern of C 1 is dominated by ascending semi-cycle from the upper reaches of the underwater plain to the estuary dam, and transits to descending semi-cycle type and asymmetrical C3 type through nearly symmetrical C2 type. This feature is consistent with the sedimentary evolution characteristics that the delta growth cycle composed of MSC 1 and MSC4 gradually increases towards the basin, and the development range and sedimentary intensity of underwater distributary channel as a reservoir framework sand body gradually expand and strengthen in each mid-term base level rising half cycle.

Figure 8-6 Mid-term Sequence Profile of Chang 6 Oil Formation

4. Long term cyclic sequence

This sequence is nearly 100 meters or even more than 100 meters thick, with Class III interface as the sequence interface. Chang 6 and its adjacent oil groups can be divided into a long-term cyclic sequence (Figure 8-3) if the top boundaries of K2, K3 and K4 are long-term lake flooding surfaces, which consists of four medium-term cyclic sequences of MSC 1 ~ MSC 4, and has an incompletely symmetric C3-type structure with accelerated lake invasion and slow lake regression, mainly with a descending semi-cycle. The regional distribution of this long-term cyclic sequence is very stable, and its low lake plane deposits are developed in the lower part of MSC 1 ascending semi-cycle, which is composed of several short-term underwater distributary channel sand bodies, representing the first growth cycle that Chang 6 Zhidan Delta began to advance to the lake basin. Later, due to the acceleration of transgression, the water body deepened into a pre-delta, with the maximum flooded surface at the top of K3 as the boundary, and the dark mudstone with thin siltstone in the upper half cycle of MSC2 constitutes the deposition in the maximum flooded period of this sequence, which is equivalent to the condensation section. The early and late sediments on the high lake level are composed of MSC2 descending half cycle superimposed with MSC3 and MSC4. The short-term underwater distributary channel sand bodies with continuous superposition of MSC3 and MSC4 ascending semi-cycle are extremely developed and have strong active progradation, while the descending semi-cycle is greatly eroded by the later channel, and the preservation is extremely incomplete. Therefore, it can represent that the Zhidan Delta has the largest advancing distance and development scale, the most developed reservoir sand bodies, the best physical properties and the best preservation in the history of Chang 6 sedimentary evolution.

It should be pointed out that the sedimentary evolution history of Zhidan Delta starts from the upper part of Chang 7 oil-bearing formation and continues to the middle and upper part of Chang 4+ Chang 5 oil-bearing formation, and there are three long-term cyclic sequences. Among them, the development of delta front deposits is mainly equivalent to the long-term decline semi-cycle stage of Chang 6 oil group to the long-term rise semi-cycle stage of Chang 4+ Chang 5 oil group, and it is also the period when the reservoir sand bodies in the underwater distributary channel of Zhidan Delta are the most developed and the combination conditions of source, reservoir and cap rock are the best.

Third, high-resolution sequence stratigraphy isochronous correlation and research significance

Taking the base-level cycle transition surface as the best time stratigraphic correlation position, the cycle sequences of Chang 6 oil-bearing formation at all levels are isochronously correlated and the corresponding time stratigraphic framework is established.

Isochronous correlation of (1) long-term cycle sequence and its research significance

In Chang 6 reservoir group, the conversion surface of long-term base level cycle includes a long-term water flooded surface with important isochronous correlation significance and two bottom and top sequence interfaces. On the basis of isochronous correlation of long-term cycle sequences marked by the above interfaces, the development positions and superposition styles of four middle-term cycle sequences are calibrated in the same long-term cycle sequence, and then the middle-term water flooded surface and sequence interface are selected as the best time stratigraphic correlation positions, and the middle-term cycle sequences with basically the same cycle structure and development amount are compared layer by layer from bottom to top, and a stratigraphic framework with long-term cycle sequences as the isochronous stratigraphic correlation unit is established (this method is also applicable to the establishment of super-stratigraphic correlation units). The stratigraphic framework of medium-long time scale established by the above method is mainly suitable for the description of source-reservoir-cap assemblage and the regional prediction and evaluation of favorable reservoir facies belts in the oil and gas exploration stage, and is also the basis for high-precision isochronous correlation of short-term and ultra-short-term cycle sequences.

(2) The isochronous correlation technology of short-term and ultra-short-term cyclic sequences and its research significance.

In view of this lake delta system, taking the dichotomy time unit of the mid-term cycle sequence as the framework, the mid-term lake flooding surface with the most isochronous significance is selected as the symmetry axis, and then each short-term cycle sequence is calibrated according to its development order and superposition style in the two time units of the mid-term base level ascending semi-cycle and descending semi-cycle. After selecting the isochronous comparison mark of short-term cycle sequence, take the symmetry axis provided by the mid-term lake flooding surface as the starting point and the bottom interface and top interface of the mid-term cycle sequence as the end point, and compare the short-term cycle sequence contained in the mid-term base level ascending semi-cycle and the short-term cycle sequence contained in the descending semi-cycle layer by layer (Figure 8-7). The comparison results are as follows:

(1) The results of layer-by-layer comparison from top to bottom or bottom to top show that the short-term cycle sequence in each position reaches the bottom interface or the top interface of the mid-term cycle sequence at the same time, which shows that the short-term cycle sequence is basically isochronous, and also shows that erosion and scouring are weak as the development factor of the mid-term cycle sequence interface.

(2) Layer-by-layer comparison of short-term cyclic sequences developed during the rising period of the mid-term base level shows that the sequences in each part cannot reach the bottom interface of the mid-term cyclic sequence at the same time, and there are one or more short-term cyclic sequences in some parts. These redundant sequences belong to the priority fillings in low-lying areas on the sequence interface, and usually consist of sand bodies under distributary channels. The greater the filling thickness, the more developed the short-term cyclic sequence, indicating that the interface of the middle-term cyclic sequence suffered from undercut erosion.

(3) Layer-by-layer comparison of the short-term cyclic sequences developed during the mid-term base level decline shows that the short-term cyclic sequences in each part cannot reach the top interface of the mid-term cyclic sequence at the same time, and one or several short-term cyclic sequences are missing in some parts, which is related to the surface exposure and erosion of the interface. Obviously, the more short-term cycle sequences are missing, the stronger the dissolution at the interface of mid-term cycle sequences, and the degree of dissolution can make use of the preferential fillers on the interface.

Figure 8-7 Isochronous tracing and comparison of small sand bodies with short cycles in sequence stratigraphic framework and time stratigraphic framework of Chang 6 oil-bearing formation in Jing 'an Oilfield, Ordos Basin.

In the isochronous correlation of short-term cyclic sequences of Chang-6 oil-bearing formation, the former case is relatively rare, while the latter two cases are relatively common. When the mid-term datum level drops to a low point below the surface, the interface will generally be eroded by different degrees of undercutting, resulting in the structure and quantity of short-term cyclic sequences developed or preserved in different parts of the delta during the same mid-term ascending or descending semi-cycle. Therefore, only by analyzing the genetic characteristics of the middle cycle sequence interface and its control on the development and preservation of the short cycle sequence in detail can we establish a high-precision stratigraphic framework with the middle cycle sequence as the framework and the short cycle sequence as the isochronous stratigraphic correlation unit. On the basis of isochronous correlation of short-term cyclic sequences, the above technical methods are also suitable for establishing a stratigraphic framework with higher accuracy, taking short-term cyclic sequences as the framework and ultra-short-term cyclic sequences as the isochronous stratigraphic correlation unit. Figure 8-7 shows the stratigraphic framework of short-term base-level sequence established by the above technical methods. The short-term and ultra-short-term time-scale stratigraphic framework established by the above method has broad application prospects in various stages of oil and gas development. It can not only be used to understand the temporal and spatial distribution law of reservoirs and interlayers in the stratigraphic framework, describe the geometric shape and relationship of reservoir sand bodies, and make more effective three-dimensional prediction and evaluation of reservoirs, but also be used to track and compare small sand bodies or single sand bodies in meters and compile large-scale isochronous sedimentary microfacies or single sand bodies distribution maps. The heterogeneity and reservoir structure of reservoir sand bodies are proved, which provides a more reliable geological model for fine description of oil and gas reservoirs, division of fluid flow units, reservoir modeling and numerical simulation of fluid flow, and even deployment or adjustment of injection-production technology.