Research content and technical route

Sequence stratigraphy is based on seismic, drilling, core and outcrop data, combined with related sedimentary environment and lithofacies, comprehensively explaining structural fluctuation, climate change and sediment input, and establishing a chronostratigraphic system of genetically related cyclic strata with stratigraphic discontinuity as the boundary. The study of sequence stratigraphy has a very realistic impact on petroleum exploration and development, especially in mature exploration areas, finding subtle oil and gas reservoirs has become the main means to increase reserves and increase production in old oil fields. The technical core of sequence stratigraphy analysis is to establish an isochronous stratigraphic framework in the whole basin. On this basis, the basin filling sequence is interpreted as different levels of sequence stratigraphic units, and the division, correlation and lateral distribution of sedimentary sequence stratigraphic units at all levels, as well as the sedimentary model, sedimentary evolution history, source-reservoir-cap assemblage, stratigraphic and lithologic traps are further studied.

Huimin sag has frequent tectonic activities and complex fault system. Due to the diachronic phenomenon of traditional stratigraphic unit division and correlation in some areas, the lateral distribution relationship between some reservoir sand bodies and oil-bearing strata series is unclear. These problems directly affect our correct understanding of basin evolution, oil and gas generation, migration, accumulation and preservation, and the overall evaluation of the whole depression. At the same time, Huimin sag still has ① low geological reserves, which are not commensurate with its area; ② The reservoir type is single, mainly structural reservoirs, lacking other types of reservoirs; ③ The relationship between tectonic evolution and stratigraphic development is not clear; ④ The degree of exploration and research is low. In addition, the theory and method of applying sequence stratigraphy theory to the study of continental stratigraphy are still immature, and there are still many problems, such as controlling factors, sedimentary datum, accommodation space and system tract division. In order to explore these problems and deepen our understanding of these problems, we have carried out the Paleogene sequence stratigraphy research in the west of the central uplift belt of Huimin sag, and combined with the problems existing in oil and gas exploration and development, determined the main research contents and technical route characteristics of this topic in the research work.

I. Main research contents

The study area is located in the west of the central uplift belt of Huimin sag, and the target intervals are the third and fourth members of Shahejie Formation of Paleogene. At the same time, the strata of Kongdian Formation are preliminarily discussed, and the specific research contents mainly include the following aspects:

1. earthquake sequence analysis

Seismic data are widely used to study the structural characteristics of the basin. With the progress of science and technology and the continuous improvement of acquisition means and processing and interpretation means, a large number of seismic data, especially high-resolution 3D seismic data, can provide us with a four-dimensional sequence stratigraphic relationship in the whole basin. Therefore, we can make full use of the characteristics of relatively low cost, large amount of data, rich content and high resolution of seismic data, and combine the geological background data and regional tectonic evolution data of the basin to establish the isochronous regional sequence stratigraphic framework of the whole basin. When establishing the sequence stratigraphic framework, we can identify the seismic reflection characteristics of the unconformity surface such as dissolution, top overshoot, top overshoot and bottom overshoot on the seismic profile, further divide the system tract according to the external geometry, internal reflection structure and various parameter characteristics of the seismic reflection wave, and then compare it with the single well data, so as to further improve the division scheme of sequence stratigraphy.

2. Logging sequence stratigraphic analysis

The resolution of logging data is much higher than that of seismic data, especially for oil fields under development. In the study of high-resolution sequence stratigraphy, a large number of logging data provide a solid material basis for this, but logging data mainly reflect the lithologic characteristics of strata. In order to ensure the isochronous division and correlation of sedimentary stratigraphic units, logging data must be limited to the isochronous sequence stratigraphic framework drawn by seismic data and geological background data. Strata with different lithology correspond to different electrical characteristics, and sedimentary stratigraphic combinations of different system tracts also correspond to logging curve combinations with different shapes. Logging data are used to identify stratigraphic units at all levels, such as sequence, system tract, quasi-sequence group and quasi-sequence, and are verified with seismic and geological data to realize high-resolution isochronous stratigraphic correlation, which provides a reliable guarantee for correctly establishing reservoir model, reservoir evaluation and development plan adjustment.

3. Sequence stratigraphic analysis of standard stratigraphic profile

Although there are abundant seismic and logging data in the extensional fault basin with complex faults in eastern China, there is often a huge defect in sequence stratigraphy analysis, that is, the natural integrity of stratigraphic relationship cannot be guaranteed. Sequence stratigraphy can explain the natural loss of strata well, but it is not enough for the formation loss caused by later faults. In addition, the continental lake basin is characterized by large degree of structural differentiation and different sequence evolution in different structural parts. Therefore, in the extensional faulted lake basin, it is necessary to first integrate the geological and seismic data of different structural parts of the whole basin with other paleontological and geochemical analysis data, establish the standard stratigraphic profile of local areas, thus correctly divide the sequence stratigraphy, and on this basis, make a lateral comparison of the sequence stratigraphy and establish the isochronous stratigraphic framework of the whole basin.

4. Biostratigraphic analysis

Traditional paleontological data used for chronostratigraphic analysis are of great significance in the study of sequence stratigraphy. By studying the loss of paleontological fossil belt, the abundance and differentiation of biota, and determining the sequence boundary and the maximum flooding surface, we can help us determine the age of the sequence and restore the lithofacies palaeogeography of the sedimentary basin.

5. Geochemical analysis

Geochemical indicators are very sensitive to sedimentary environment. By studying the analysis data of major elements, trace elements, stable isotopes, rare earth elements and organic geochemistry, it can also help to explain the boundary and internal characteristics of sequences and system tracts, as well as the recovery of paleotemperature, paleosalinity and redox degree.

6. Comprehensive sequence stratigraphic analysis

Based on the comprehensive geological, seismic, drilling, logging, core, paleontology and geochemical data, through mutual verification, modification and perfection, the isochronous sequence stratigraphic framework is established in the whole basin, stratigraphic units at all levels are divided, and the lateral isochronous correlation is carried out, so as to further restore the structural evolution history and sedimentary filling history of the basin, realize the reconstruction of lithofacies palaeogeography, evaluate the combination of source, reservoir and cap rocks, and predict favorable oil and gas accumulation zones.

7. Study on sequence stratigraphic model

Based on the sequence stratigraphy analysis of Paleogene Shahejie Formation and Kongdian Formation in Huimin Depression, the dynamic mechanism and controlling factors of sequence evolution of continental extensional rift lake basin are analyzed, and the influences of tectonic subsidence, climate change, sediment supply and lake level fluctuation on sequence development and evolution are re-recognized. The concepts of sedimentary base level and accommodation space are preliminarily discussed, and the sequence evolution models of continental lake basins in different evolution stages are summarized.

Second, the technical route

Based on the basic principles and methods of sequence stratigraphy, firstly, according to the regional geological background and seismic data, referring to the sedimentary stratigraphic cycle law of adjacent areas, a sequence stratigraphic framework is established in the whole basin. Under the constraint of this sequence stratigraphic framework, system tracts, parasequence groups and parasequences in the sequence are identified and divided on the divided standard sections by using logging data, core data, paleontological data and geochemical analysis data. And mutual verification, modification and improvement with seismic data and geological data. Then, from the standard profile, from point to surface, from point to surface combination, from plane to profile combination, some problems are carefully scrutinized and repeatedly demonstrated, thus establishing the isochronous sequence stratigraphic framework of the whole basin, and integrating geological, seismic, logging, core, paleontology and geochemical data, according to the basic theories and methods of sedimentary petrology, stratigraphy, tectonic geology, sedimentary geochemistry, paleontology and petroleum geology, The paleogeography, paleoclimate and paleoenvironment of the basin are restored, the sequence types, genetic mechanisms and controlling factors are analyzed in detail, the evolution model of sequence stratigraphy of continental lake basin and its control law on source, reservoir and cap rocks are summarized, and the favorable source, reservoir and cap rocks are evaluated. See figure 1-3 for the specific research flow.

Figure 1-3 Research Methods and Technical Roadmap