Application of 3D Seismic Acquisition Technology in the Complex Geological Structural Zone of the Ordos Basin

The thrust structures in the central section of the western margin tectonic belt of the Ordos Basin are well developed, with diverse types of fault-anticline-syncline oil and gas reservoirs in the Mesozoic strata. The interference of reflection waves from Mesozoic reservoirs is significant, the reflection waves from the Upper Paleozoic Permian tight gas reservoirs are weak, and the Ordovician Wulalik shale gas reservoir with rich cracks has a burial depth greater than 6′000 m. The reef and shoal bodies in the Kelimoli Formation are regular in shape. These primary geological targets pose high risks for oil and gas 3D seismic exploration. The underground geological target layers are numerous and complex; the development of high-steep structures above the main Permian detachment zone makes seismic reflection focusing imaging difficult. The surface sandy soil is dry, with high porosity and low seismic wave velocity, thick deceleration layer, presence of gypsum interlayers, rapid seismic wave attenuation, and rapid changes in lithology, thickness, and speed of the underlying old strata in both horizontal and vertical directions, affecting the quality of seismic data acquisition. Previous two-dimensional and three-dimensional observation azimuths were narrow, with an aspect ratio of only 0.4 and a gun density less than 200000 channels per square kilometer. The resulting data imaging precision is low and cannot meet the requirements for accurate identification of geological targets in shallow, intermediate, and deep strata, as well as complex structures for three-dimensional exploration needs. Addressing the challenges of seismic exploration in the study area due to its “dual complexity” of surface and subterranean structures, based on the analysis of three-dimensional seismic methods in neighboring eastern areas and considering the geological target difficulties of this area, for the first time in this region, a method and technology were adopted that involves “wide azimuth, high-density three-dimensional observation system; EV56 wide frequency high precision controllable seismic source excitation; 5Hz wide frequency high sensitivity single detector node instrument high fidelity reception”. The seismic data collected had its dominant frequency increased by 5 Hz, and the signal-to-noise ratio improved by 3. The imaging of complex structures is clear, the quality of fold data is good, and the reflection axis of the Wulalic Formation shale in the deep Paleozoic is continuous. The morphology of the reef and shoal bodies in the Kelimoli Formation platform facies belt can be distinguished. This method and technology provide three-dimensional seismic acquisition techniques for systematic target exploration in “the complex surface seismic exploration conditions and the deep geological structure” areas with high steep structures and deep marine shales, pointing out a new direction for efficient exploration.