Seismic Sedimentology And Sand Bodies Prediction Of The Cretaceous In Baiyinchagan Sag

The Baiyinchagan Sag is characterized by multiple provenance types,narrow sedimentary facies and rapid change of sand body types.The early exploration targets are sandbodies with stable development and wide distribution in the basin margin.With the exploration target penetrating into the basin,the uncertainty of sand body size and distribution increases,which puts forward higher requirements for favorable sand body prediction.Aiming at finding favorable sand bodies,this paper studies the development law of sand bodies in the isochronal sequence stratigraphic framework by comprehensive application of drilling,logging,seismic and analytical laboratory data,and clarifies the characteristics of provenance distribution,sedimentary system characteristics and sedimentary facies development mode in the depression.On this basis,the distribution of various sand bodies in the sag is predicted by using seismic sedimentology and geophysical techniques.The main findings and conclusions of the paper are as follows.Based on the reflection characteristics,sedimentological characteristics and typical single well analysis of seismic profiles,the key sequence boundaries of the Cretaceous in the study area are identified,and the basis for the division of sequence at different levels is determined.Through typical single well sequence division and well-seismic combination tracing and correlation interpretation,four third-order sequences are divided in Cretaceous,and four sequence stratigraphic development modes are summarized.Based on regional outcrops,distribution of bedrock in the basin,rock composition,heavy minerals and palaeogeomorphological analysis,the relationship between palaeogeomorphological characteristics and ancient provenances and sedimentary systems is discussed.It is confirmed that the surrounding outcrop and the bedrock area extending into the basin are the main provenance areas of various sediments in the basin,and the sediments in the basin are divided into six major source areas.They are Xilinhaolai,Daerqidong,Wengte,Sanghe,Guer and Guerdong.On this basis,three types of ancient valleys are identified through the analysis of palaeogeomorphology and the characterization of ancient valleys.The capacity of carrying and unloading sand bodies of various valleys is semi-quantitatively analyzed.It is clear that "W" type valleys have large sand control area and "U" type valleys have the characteristics of long haul distance.Based on a large number of core observation,well logging and seismic facies research,five types of sedimentary facies were identified in Baiyinchagan Sag,including braided river delta,fan delta,turbidite fan,beach bar and lake.It was determined that large braided river delta-lake sedimentary system was mainly developed in the gentle slope zone in the south of the Sag,fan delta-lake sedimentary system was developed in the northern steep slope zone,and turbidite was mainly developed in the interior of the Sag.The fan-lake sedimentary system and the sedimentary evolution model of the basin from strong fault depression to depression are established.Through geophysical model test and numerical forward simulation analysis,the seismic response characteristics of sand and mudstone formation combination are further studied,and the geological models of different types of sand bodies are established.On this basis,the high-precision prediction of delta,turbidite fan and beach bar sand under the control of sequence framework is realized by means of 90 degree phase conversion,seismic sedimentology,stratum slicing technology and time-frequency analysis technology,combining with the forward simulation results of seismic attributes,key well data,sedimentary model,palaeogeomorphology and geological model.