The Characteristic And Evolution Of Sequence-lithofacies Palaeogeography Of Lower Paleozoic Around Bohai Gulf Area

The lower Paleozoic is a set of relatively stable marine carbonate rocks deposition around Bohai Gulf area. For many years many researchers have studied the area from each aspect in detail, but not yet obtains the significant breakthrough in the oil gas exploration. This thesis according to the theory of sedimentology and sequence stratigraphy and so on, taking the sequence evolution, deposition evolution and basin evolution as the master line. On the basis of the field section, the region stratum material, the generalized analysis of drill-core and log information, then dividing and contrasting the sequence stratigraphy of the lower Paleozoic around Bohai Gulf area, then establishing sequence stratigraphic framework. With the combination of point and face, working out sequence lithofacies palaeogeography map and studying sequence palaeogeographic evolution and distribution of various periods in order to provide reference to oil-gas exploration. The main achievement and the understanding are generalized as follows:(1)Using the modem stratigraphy multiple stratum division and contrast principle to systematically and comprehensively clean up and contrast the lower Paleozoic in study area.(2)Basing on the massive field sections observation and the drill-core description in detailed, eight types sequence bottom boundary were identified, the sequence surface can be summed up four genetic types which are the orogenic erosional sequence unconformity, the uplift erosional sequence unconformity, the transgressive onlap sequence unconformity and the exposed sequence unconformity.(3)The lower Paleozoic Cambrian can be partitioned into 13 third-order sequences and 2 supersequences(the second-order), and the Ordovician can be partitioned into 11 third-order sequences and 2 supersequences. The basic characteristic of sequence stratigraphy unit, thestacking relations and the spatial distribution rule have been analysised detailedly and then established isochronal sequence stratigraphy framework of the lower paleozoic in the study area.(4)According to the sea-level change, the deposit supply, the tectonic subsidence, the climate and so on, the main controlling factors of sequence development is discussed in this thesis, then revealed the primary control function of the sea-level change to the sequence development and the depositional system tract distribution.(5)Nine supersequence lithofacies palaeogeography maps in the study area lower Paleozoic have been compiled on condition of supersequence system tract used as drawing unit, it reveals the characteristic and evolution of lithofacies palaeogeography of different system tract period; In view of the key formation of lower paleozoic, seven lithofacies palaeogeography maps have been compiled on condition of third-order sequence used as drawing unit, all of these palaeogeography maps have characteristic of isochronism, success of the sedimentation process and practicality.(6)Through analysis to the distributed rule and the combination characteristic of hydrocarbon source rock, the reservoir and the cap rock in the supersequence stratigraphic framework, it is believe that the fresh hydrocarbon-generation potential and the reservoir quality of Ordovician are better than that of Cambrian in the study area.(7)In the light of analysis the geologic characteristic of Ordovician carbonate rocks buried hill hydrocarbon reservoir, the transformation function of the later period tectonic movement to the Ordovician system reservoir is discussed.