THE DEPOSITIONAL FEATURE OF MIXED SYSTEM IN MARINE ROCKS OF CHANGCHENG SYSTEM OF PRE-CAMBRIAN IN NORTHERN CHINA

The Proterozoic marine deposits of Changcheng system of pre-Cambrian in Northern China including two outcrops from Beijing YQ area and Shangxi province, produced in ca 1.7 billion to 1.4 years ago, developed multi-cycle mixed system of clastic-carbonate rocks. The paper aims at determining systems tract of sequence, stacking parasequences and building up depositional model of mixed carbonate system for predicting high-quality reservoirs of deep strata in pre-Cambrian.

Based on detailed observation of twelve outcrops named Qiaotou and Zhongtiaoshan profile and so on, description of 25 samples, and microscopic analysis of thin section and SEM on 22 samples, the depositional model of marine mixed carbonate rock of Changcheng System in Yanqing area of Beijing were set up. The marine quartz sandstones of Changcheng System, located at lower part of sequence, have parallel beddings and low-angle waving cross beddings，as well as a lot of ripples. The contents are composed of main quartz mineral, and less than 20% feldspar and rock debris. The marine quartz sandstones of Changcheng System derived from mainly weathering products of granite mother rocks. Therefore, it has been inferred that the marine quartz sandstones of Changcheng System were formed in shore and shallow marine sandy environment. There developed pores between grains to be potential oil reservoirs, especially in unfilled part of sparstone dolomite in hybrid rocks.

In summary, Changcheng System underwent transgressive process with slower rapidity and bigger rising extent. The continuous rising sea level leads to environment changes from shore and shallow marine to carbonate platform. These mixed carbonate system of shoreline beach and open platform may form important high-quality reservoirs and petroleum exploration targets.