Sequence Stratigraphic Framework of Wuliyasitai Faulted Basin in Erlian Basin Group and its Episodic Filling Evolution

The Wuliyasitai faulted basin,a typical faulted basin in Erlian basin group,is bounded by a listric normal fault on the northwestern side of the basin,with very thick marginal fan complexes indicating contemporaneous sedimentation along the faults.It is infilled with Jurrasic and Early Cretaceous sediments of up to 5 000 m in thickness which a series of different scale hiatuses and tectonic events have been discerned during hydrocarbon exploration in the basin.In this present paper the principle and method of high-resolution sequence stratigraphy are adopted to deal with seismic reflection profiles and well logs data.the various orders of sequence boundaries have been identified and the sequence stratigraphic framework of Wuliyasitai faulted basin has been reconstructed.According to regional and local unconformity and corresponding conformities,the basin infillings of the basin may be subdivided into two tectonic sequences,represented by J 1～J 2 (TC1)and K 1(TC2)rifting type filling sequences,respectively,which constituted two proto-type basins developing in two rifting stages,among which the major infilling sequence TC2 may be further subdivided into four sequence sets and eleven sequences.Generally,the boundaries of tectonic sequences were regional unconformities,and the unconformity boundaries of sequence sets can be traced on a basin scale ,while the unconformity boundaries of sequences only developed in basin margins or intrabasin uplift,which merged into conformity boundaries toward center part of basin.Based on the sequence stratigraphic framework,sequence system tracts that are characterized by one-lay type system tract,often developed in initial and final evolution phrases of basin,and the two-lay type system tracts consisting of fining-upward tract,i.e,expanding lacustrine system tract and coarsening upward tract,i.e,contraction lacustrine system tract have been reconstructed.Unlike depositional systems of sea basin, in lacustrine basin like the Wuliyasitai faulted basin subaqueous fan, deep lake turbidite fan and fan delta developed generally in highstand phrase of lake level. Basin tectonic subsidence rate and total subsidence rate computed by backstripping method show three changes from fast to slow,which corresponds with three sequence sets stated above,indicating that the basin underwent thee stretching episodes during synrifting stage,which have been interpreted asa result of a multiple stretching of the lithosphere.The episodic tectonism of the basin,showing clear relationship to depositional fillings of the basin,is a major controlling factor of the formation of higher-order sequences in the basin,such as tectonic sequence and sequence sets,and controlled the development of sequences to a certain content.The conclusions have been applied to the prospect of reservoir and hydrocarbon source-rock distribution and lithologic-stratigraphic traps in this basin,and proved to be effective.