A Research on the Techniques for Identifying Lithologic Stratigraphic Traps in the Southwest 3D Survey of Chaidamu Basin
The Southwest Chaidamu 3D survey is located between Kunlun and Aerjin mountains in the south region of west Chaidamu basin, which had been proved to be one of the most promising areas in this basin. The survey area is situated at the edge of the Basin. The sand bodies originated from Kunlun and Aerjin mountains are good for forming various kinds of lithologic stratigraphic traps. Nevertheless, the thinness of oil-baring layers, severe facies change and the weak seismic or sonic responses characterized by this area are the geological factors that hinder the exploration of lithologic stratigraphic reserviors. And what is more, the lack of effective geophysical techniques and systematical research strategy is one of the most important reasons.
Aiming at the geological characteristics and the current exploration situation in the Southwest Chaidamu 3D survey area, lithologic stratigraphic reservior identification and analysis work was conducted. During the research, the contemporary tectonics and tectonic evolution had been studied to analyze a relationship between tectonics activity and lithologic traps form and development.
The study of palaeo-structure not only provide the foundation for the stratigraphic sequence model building and, systems tract deposition model division and the establishment of facies classification, but importantly determine a favorable zone for the identification of lithologic and stratigraphic traps.
An isochronous stratigraphic framework was established through the study of sequence stratigraphy in the study area. The sequence developing at Tertiary in study area was divided into threeⅡsequences and sevenⅢsequences. Several favorable lithologic stratigraphic zones had been found and determined variable traps to make clearly next research.
Different sedimentary facies types had been found through studying sedimentary facies. The various cycles of reservoirs distribution and reservoirs nature had been determined and then provided evidence for finding high quality good reservoir.
The identification techniques and methods had been studied for lithologic stratigraphic traps to establish models of identification and analysis for Southwest Chaidamu Basin exploration basing on geophysical analysis. And the better effect had been obtained in practice.
The targets had been optimized to provide drilling suggestion basing on favorable zones and lithostratigraphic traps evaluation. The better effect and economic effect had been achieved through drilling.
Several innovations had been obtained through studying the project.
1 the theory and methods of "four determinations and one optimization" had been proposed first theorically for lithostratigraphic strata study.
In this project research, the theory of "four determinations and one optimization" had been first proposed theorically for lithologic traps study through analyzing lithologic stratigraphic trap geologic features, geophysical characteristics and seismic identificaiton and analysis combining with traps geologic evaluation, which are consisted by ancient structural analysis to determine trap favorable development areas, sequence stratigraphics study to determine the vertical favorable areas, sedimentary facies to find reservoir development law, seismic technology to identify and determine traps and comprehensive evaluation and target optimization.
Compared with conventional lithological trap identification methods, the method of "four determination and one optimization" is to analyze ancient structure which is essential for lithologic reservoirs exploration. The results obtained in the practical application were very good.
During palaeo-structure study, the strata dip correction had first been proposed to recover the true strata thickness. At the same time the strata erosion thickness recovery method was first used to make maps of palaeo-structure in Southwest Chaidamu Basin. At present the sets of palaeo-structure maps had become essential basic maps and played an important role in ithostratigraphic reservoirs study.
2 four identificaiton models had been estabished first and the corresponding techniques and methods had been developed.
During the project implementation, on the basis of study of geologic features of lithologic stratigraphic traps, four identification models had been established according to sedimentary enviornment, traps types, main controlling factors and geophysical features. Each model had been used to identify and analyze the lithostratigraphic traps and the specific technique had been built for lithologic traps identificaiton. Four technical series had been developed, which are seismic reservoir prediction based on multiwavelet seismic trace decomposition, prediction of high quality reservoirs with great thickness and good physical property, fan bodies identification and shape configuring combining ancient structural analysis with seismic facies analysis, stratigraphic overlap line coherence strengthening and peak(trough) number technique and quantitative analysis of glutenite thickness. Among these techniques, the fan bodies identification and shape configuring combining ancient structural analysis with seismic facies analysis and stratigraphic overlap line coherence strengthening and peak(trough) number technique were first developed and used. The seismic reservoir prediction based on multiwavelet seismic trace decomposition, prediction of high quality reservoirs with great thickness and good physical property were also first used and proposed.
3 The significant exploration results had been otained through the techniques application. The drilling results were great.
In the project study and application, the rich geologic results and better exploration effects had been obtained. In 2009 an oilfield with predicted reserves of hundred million ton, which had been found in Kunbei area in Chaidamu Basin should be contributed to lithologic and stratigraphic oil and gas reservoir studies, its reserve was accountedfor 43% of total reserves.
The theory and methods of "four determinations and one optimization" proposed in the project had not only been applied best to the area, but also to the west of the Basin and the margin of north Basin, which had showed a good promotional value.
In the project study and application, the rich geologic results and better exploration effects had been obtained. There were 137 lithostratigraphic traps having been found in Southwest Chaidamu Basin 3D area with traps area of 946.4km2. The proposed well were 72, among which 53 wells were adopted, and 23 wells had drilled industrial oil. The wells aimed had increased from 2-3 planed in 2006 to 27 planed in 2009, which was occupied from 13.3% to 70%.
At same time when the project studied, the new discovered trap area was 94km2, the predicted hydrocarbon reserves was 5376×104t, the new controlled area bearing-hydrocarbon was 67.1km2, the new controlled reserves was 3566×104t, the new proven oil-bearing area of 44.85km2 and proven reserves was 2038.08×104t.
This research project achieved the expected goals.