STUDY ON THE SECONDARY PORES OF SANDSTONE OF THE LOWER TERTIARY IN DONGYING DEPRESSION, SHANDONG

The reservoirs of sandstones of the Lower Tertiary in Dongying Degression,Shandong were proved to be quite developed.The writer tried to determine the structure type, genesis, abundance, distridution of secondary pores and their controlling factors so as to distinguish the varying sequence of diagenesis and its effect on secondary pores by the careful observation of hand specimen, thin sections of the rock, cast thin section, the natural plane of fracture, rounded face of the sample and the surface of the sand grain using stero-microscope, polarizing microscope and scanning electron-microscope. It is suggested that the marks to identify the secondary pores of sandstone in this area are as follows: partial dissolution, corroded grain, overquantity of pores, leaked skeletons, moldic pores, attaching-granular pores, residual carbonate cements, elongate pores,inhomogeneity of packing, oversized pores,immersion carbonate matrix or cement, crashed grains, gaps of rock etc. Secondary pores in this araea consist of seven types of structures, such as secondary intergranular pores,moldic pores,attaching-granular pores, intra-constituent pores, oversized pores, intergranular intercrystal pores and fracture pores. The first five of them result from the dissolution in deep subground with the largest amount,widest distribution and they are of great significance;while fracture pores brought about by overying pressure and structure action, and intercrystal pores are caused by constriction of recrystal in carbonate matrix. These two types of pores have less amount and are of little importance. The secondary pores of the lower Tertiary sandstone in this area are not so well developed as the primary ones,but they have a regular distribution they develop better in north then in south, and better in the deeps than in shallows. The lateral distribution of secondary pores is controlled by the structure of the basin while vertical distribution is determined by the combination of sedimentary facies and the mineral composition and texture and structure of sandstone have somesthing to do with the development of secondary pores. In a word, the developed faults and the reservoirs close to the source rocks and the massive coarser sandstone rich in dissoluble compositions provide a suitable condition for the development of seconday pores. There is a close relationship between secondary pores and diagenesis. The complex diagenetic change of the rocks especially the early cementation of mosaic Fe-calcite, replacement of carbonate and dedolomitization in the depth were indirectly suitable for forming secondary pores. Mechanical compaction could result in little amount of pores and fractures, but it mainly destoried the pores which were formed before. The transformation of clay minerals and the dissolution of the acid water produced by thermal maturity of organic mattter were the main factors of forming secondry pores. It is concluded that 1 ), secondary pores occur in many oil reservirs, of which some are predomnant with secondary pores, so secondary pores are the main parts of the sandstone reservoirs in this areas 2), secondary pores concentrated over the threshold value pores become the path along which hydrocarbon migrate upward, this provides a useful information for the determination of threshold value; 3), secondary pores deepen the effective pores of sandstone, based on which the guideline could be given and the exploration of the oil formation near source rocks should be made; 4), the reservoirs with secondary pores have developed carbonate cements, larger mechanical strength, a better results of acidization and the connection of pores caused by long-term flow of acid water are advantageous to the development of the oil field.