This paper discusses how to explore oil and gas in sedimentary basins.

The process of oil and gas exploration can be divided into several stages, each of which is independent and continuous. The relationship between oil and gas exploration stages and work sequence is usually called exploration procedure. From exploration to development, an oil and gas field should follow scientific exploration procedures and adopt comprehensive exploration methods to find out the basic underground geological conditions, find out the distribution law of oil and gas, find oil and gas fields and find out oil and gas reserves.

Oil and gas exploration procedures mainly follow the hierarchical structure of oil and gas exploration evaluation objects. The traditional classification of oil and gas exploration objects in China is based on geological structural units, or structures. The basic theoretical basis is that structure controls sedimentation and structure also controls crustal deformation. With the in-depth development of oil and gas exploration, the evaluation of exploration objects is getting deeper and deeper, and the traditional evaluation system of oil and gas exploration in China is gradually improved, which can be roughly divided into five levels: structural area-oil-bearing area, sedimentary basin-oil-bearing basin, depression and uplift, depression and uplift, secondary structural zone, belt, oil and gas accumulation zone and trap.

Since 1980s, major western oil companies and academic circles have gradually established a basically unified oil and gas exploration and evaluation system.

The four different exploration stages proposed by L B Magoon and W.G.Dow are sedimentary basin, petroleum system, reservoir-forming assemblage and prospective trap. The former two do not involve the content of economic evaluation, while the latter two are closely related to economic evaluation.

The evaluation procedures of major western oil companies can be represented by Exxon's evaluation system:

Tectonic stratigraphic province

↓

basin

↓

Petroleum system

↓

Reservoir-forming assemblage/exploration assemblage

↓

Prospect/field

In addition, P.A.Allen( 1990) and others put forward the distribution channel between petroleum system and reservoir-forming assemblage, and some people also adopted the distribution trend order between reservoir-forming assemblage and prospective trap.

With the continuous exchange of petroleum geological theories between China and the West, the western concept of oil and gas evaluation system has also entered China, and many oil and gas exploration evaluation systems have been produced in combination with the summary of exploration experience in China. Among them, Wu Shoucheng put forward four-level evaluation of oil-bearing area, basin, belt and trap in 1994. Three-level evaluation of basin, trap and reservoir proposed by Ding Guiming in 1996; The sequence of oil-bearing geological units proposed by Hu Jianyi in 1997 includes five levels, namely, oil-bearing structural sedimentary system, oil-bearing basin, oil-bearing system, oil-bearing accumulation zone and oil and gas reservoir. In 2000, Tong believed that the oil and gas exploration and evaluation system should be composed of six levels, namely, geotectonic-stratigraphic region, basin, petroleum system, reservoir-forming assemblage, reservoir-forming belt and prospective trap.

1? Tectonic stratigraphic area

As the highest evaluation unit, the characteristics of areas with similar plate tectonic evolution history control the formation and distribution of basins and the tectonic-sedimentary types of basins, which can be used for the selection of large areas, especially in transnational operations.

2? basin

Basin is the basic unit of evaluation, which is composed of one or several prototype basins superimposed and/or compounded. The basin has the same genesis and structural style, and has a basically unified history of sedimentary filling and deformation. The generation, migration and accumulation of oil and gas all occur in the basin. In the selection of exploration object, basin is more important than geotectonic-stratigraphic area, so many geologists advocate that the initial unit of exploration object evaluation is basin. But the basin mentioned here is broad and the definition is not strict. As a sedimentary unit, depressions and depressions can also be regarded as basins. Although there is no quantitative economic evaluation in basin evaluation, the primary purpose of evaluation is still to optimize the basin, not only to make a long-term evaluation of oil and gas potential, but also to determine the size of oil and gas fields and the high output of single wells on the basis of basin location, operating conditions and buried depth of oil and gas reservoirs.

3? Petroleum system

It is an oil-gas generation-migration-accumulation unit of source beds. In the simplest basin, a basin has only one petroleum system; A complex basin has several effective source rocks and/or hydrocarbon generating centers, so a basin has several petroleum systems. A complex basin composed of multiple prototype basins must have multiple petroleum systems. For superimposed basins composed of a single prototype basin, the evaluation of petroleum system should start with a single petroleum system, and then combine several petroleum systems with similar history of generation, migration and accumulation into a composite petroleum system for unified evaluation. The petroleum systems in a basin composed of several prototype basins should also be combined according to prototype basins on the basis of the evaluation of a single petroleum system, and then the influence relationship between petroleum systems in superimposed and/or planar parallel composite prototype basins should be studied and evaluated.

The evaluation of petroleum system is mainly to determine the scale and time of oil and gas generation, the stratigraphic range and geographical range of oil and gas migration and accumulation.

4? Reservoir-forming assemblage

Reservoir-forming assemblage is the basic unit of oil and gas accumulation. The oil and gas in the reservoir-forming combination can come from one oil-bearing system or from several oil-bearing systems, and one oil-bearing system can provide oil and gas to several reservoir-forming combinations. Under the condition of oil source, the stratigraphic distribution of oil and gas accumulation is first controlled by regional caprock and then by reservoir. The trap type determines the pattern of oil and gas accumulation. It can be seen that the reservoir-forming assemblage mainly evaluates the vertical distribution of oil and gas in the basin.

In basins with single petroleum system and single reservoir-cap assemblage, the reservoir-forming assemblage is relatively simple (such as Nanxiang Basin). Basins with single or multiple petroleum systems but multiple reservoir-cap combinations have formed multiple reservoir-forming combinations through vertical migration of oil and gas, and each reservoir-forming combination has its own oil-gas accumulation laws and characteristics, so the evaluation of reservoir-forming combinations is of great significance. For a composite superimposed basin composed of several prototype basins, it is more important to distinguish the laws and characteristics of oil and gas accumulation in various reservoir-forming combinations, so the evaluation of reservoir-forming combinations is of greater significance. The reservoir-forming combination lays a foundation for quantitative resource evaluation, risk analysis and economic evaluation.

5? Reservoir-forming zone

Reservoir-forming assemblages can be widely distributed in the basin, but the oil and gas in each reservoir-forming assemblage are always enriched in some parts, that is, favorable reservoir-forming belts, such as structural belts, stratigraphic pinch-out belts or overlapping belts, which is the combination of reservoir-forming assemblages and traditional zoning evaluation in China.

6? Expected trap

The role and significance of Yuan Jing trap evaluation in the oil and gas exploration and evaluation system are relatively unified, so I won't repeat them here.

For oil companies, from the evaluation of large areas, basins, petroleum-bearing systems and reservoir-forming combinations, the focus is on reservoir-forming combinations. In the early stage, the reservoir-forming combination is predicted by analogy according to the geological background, and the reservoir-forming combination is evaluated by evaluating and predicting key geological parameters in the later stage. As for the prospective trap stage, it depends on the evaluation and prediction of various geological parameters.

The evaluation system is established according to objective geological laws, and there are exploration units (or levels) from large to small. However, the actual exploration objects that are common in the world are blocks defined by geographical coordinates, and they are not necessarily related to exploration units. Generally speaking, a block is a part of a basin, which can include all sub-evaluation units or a part of them. As an evaluation system, it should be based on the above levels, but the actual exploration operation is limited to blocks. The evaluation of basin and petroleum system is the basis of block evaluation, and the focus of block evaluation is the reservoir-forming combination, reservoir-forming belt and prospective trap contained in the block.

Second, the specific tasks and work steps in different exploration stages

Due to the differences in exploration management system and specific geological background, different countries adopt different oil and gas exploration procedures. The following mainly introduces the exploration procedures of the former China Oil and Gas Corporation. The exploration plan divides oil and gas exploration into three stages: regional exploration, trap pre-exploration and oil and gas field evaluation and exploration.

1? Regional exploration

The main geological task of regional exploration is to make an overall geological survey of the basin, depression (depression) or part of it, find out the basic conditions of regional geology and petroleum geology, make early oil and gas prospect evaluation and resource estimation, select the most favorable depression (depression) and structural belt, and put forward the pre-exploration scheme.

The work steps in the regional exploration stage are divided into four steps: project establishment, geophysical survey, parametric drilling and basin evaluation. The establishment and design of regional exploration projects are based on the long-term strategic arrangement of oil and gas exploration in the whole country and the whole region, and efforts are made to ensure the sustained growth of oil and gas reserves while maintaining the existing production level; Geophysical survey is to carry out geophysical exploration extensively in the selected basin, further study the regional geological structure of the basin, divide the structural units, find out the secondary structural zones, and put forward the deployment scheme of parameter wells; After mastering the general situation of regional structure and secondary structural belt and predicting the distribution of facies belt and sand body, parameter wells can be deployed for drilling; Basin (depression) evaluation is to comprehensively apply various data and information such as ground geological survey, geophysical and geochemical exploration, drilling, logging, testing and analysis to establish a basin geological model, estimate oil and gas resources and their two-dimensional spatial distribution in the basin, and optimize favorable oil and gas-bearing zones.

2? Trap pre-exploration

The task of trap pre-exploration is to find commercial oil and gas fields and prepare for oil and gas field evaluation and exploration.

The work steps of trap pre-exploration stage are divided into: determining pre-exploration items, detailed seismic survey, pre-exploration well drilling and trap evaluation. In the later stage of regional exploration, the pre-exploration projects are determined on the basis of local structural evaluation, and the traps with the most prospect and economic value are selected for pre-exploration; Seismic detailed survey is a rapid seismic detailed survey of selected circles, and the density of survey lines is 1km × 1km and 2km×2km. In the application, the obtained data are specially processed, and the lithology and seismic stratigraphy are studied to find out the distribution of permeable layer (sand body) in the structure. Pre-drilling of exploratory wells can be divided into determination of exploratory well location, exploratory well design and scientific exploratory well. Trap evaluation includes analyzing the sealing conditions, size and height of traps, determining the main oil-bearing series and types of oil and gas reservoirs, predicting the productivity of oil and gas reservoirs, providing prediction, control and proven reserves, and providing drilling schemes for oil and gas field evaluation.

3? Oil and gas field evaluation

The task of oil and gas field evaluation and exploration is to evaluate the discovered oil and gas reservoirs in the pre-exploration stage, and has now entered the oil and gas field evaluation and exploration stage. The core task of this stage is to find out the commercial value of the discovered oil and gas fields with as few exploration wells as possible, and obtain all the first-hand information in the shortest time to prepare for development.

The steps of oil and gas field evaluation and exploration are divided into project establishment, seismic survey, evaluation well drilling and reservoir evaluation. Project initiation refers to geological analysis and evaluation of the structure, calculation of expected workload and effect, and determination of technical measures after the exploration project is put forward. The oil company will organize the project initiation and approval, and then report it to the headquarters for approval. Fine seismic survey is to arrange three-dimensional seismic or 0.5km×0.5km accuracy for complex reservoirs, and meet the accuracy requirements of final mapping scale 1: 25000 or 1: 10000, with emphasis on structural interpretation, reservoir interpretation and oil and gas detection. When drilling appraisal wells, the well spacing is generally between 1 ~ 2.5km. Oil and gas reservoir evaluation is mainly to carry out oil and gas reservoir description research.

Rolling exploration and development is carried out for complex reservoirs, that is, while developing the basic proven areas, new fault blocks are continuously drilled, edges are expanded and new strata are deepened.