Abstract:

This paper analyses the type and evolution of primary pores, the genesis of secondary pores and the factors of controlling their distribution in the Paleogene clastic reservoirs in Dongying sag. Study of various data,including thin section analysis,SEM,cathodoluminescense,poroperm characteristics,carbonate content and other data,shows that the pores in Paleogene clastic reservoirs in Dongying sag experienced the evolutionary process from primary to secondary pores,i.e.those occur at a depth less than 1 650 m are mainly primary pores;those occur between 1 650m and 1 900 m are the combination or coexistence of solution and primary pores,due to the relatively weak solution action; while at a depth of over 1 900 m, solution and cementation would play the main role,which result in to be mainly of secondary pores. In different areas and buried depths, secondary pores appear to have been developed differently: secondary pores mainly develop at a depth of 1 650～2 450 m on the steep slope zone in the north, especially best develop at 2 100 m in central uplift zone,secondary pores have a relatively good development at a depth of over 1 600 m,with the best development in the interval of 1 850～2 500 m on the gentle slop in the south,secondary pores are generally developed at a depth of 1 900～2 600 m.As a whole,the development of secondary pores tends to be deeper and deeper from north to south,but the extent is apparently become weaker and weaker.All these have mainly been affected by the different reservoir facies, paleo-geothermal gradient and organic maturation in different areas,as well as by the distances of the areas from the oil-generating sag and fault zone. The formation of secondary pores has mainly been related with the solution of carbonate and feldspar, and the dehydration of clay minerals; in other words,it is the result of inorganic diagenesis and the soluble constituents to be dissolved by the carbon dioxide and organic acid during the organic maturation. According to thin section observation, most secondary pores have been formed from the dissolution of intergranular carbonate cement, which has been proved by the negative correlation between porosity & permeability and carbonate content. The dissolved carbonate materials consist of calcites formed in early diagenesis and ferrocalcite & ferrodolomite formed in late diagenesis. Part of secondary pores has been created by the dissolution of feldspars (including plagioclase and potash feldspar) and lithoclasts of intermediate-basic volcanic rocks. In addition, the main depth that montmorillonite to be dehydrated its interlayer water and to be transformed into mixed-layer illite& montmorillonite and chlorite is consistent with the depth interval of secondary pores. The low salinity of interlayer water dehydrated from clay minerals contributes to the further solution of carbonate and feldspar. Secondary pores of clastic reservoirs in the study area have the following distribution characteristics. (1) Secondary pores have been controlled vertically by the time of source rock maturation,i.e.have been controlled by the burial depth(or geotemperature) of sandstones. (2)Secondary pores have been controlled horizontally by the location of sources acidic water. Secondary pores are less developed in the areas far from the oil-generating center. The oil-generating center has been located in Lijin depression in the northern part of Dongying sag, so secondary pores have better development in Binnan, Lijin and Shengtuo areas on the steep slope zone in the north than those on gentle slope zone in the south. (3)Secondary pores are closely related to the sedimentary facies zone, i.e. sandstones deposited in subaqueous environment (strong hydrodynamic condition) have better development of secondary pores than those deposited in terrestrial environment (weak hydrodynamic condition).(4) Secondary pores are more developed in the sandstones near the fault zone. Strong movement of fault would not only produce fissures in the sandstones, but would also become good pathways of acidic water.The sandstones near the pathways have long been affected by acidic water, so there have been more opportunities to be dissolved into secondary pores. For example, the steep slop zone in the north has better development of secondary pores than the gentle slope zone in the south has, because there are more faults on the steep slope zone.

Key words: clastic reservoir rock, pore type and its evolution, genesis of secondary pores, Dongying sag