THE CHARACTERISTICS AND GEOLOGICAL IMPLICATIONS OF ANALCITE IN LACUSTRINE SHALE OF THE SECOND MEMBER OF KONGDIAN FORMATION IN CANGDONG SAG, HUANGHUA DEPRESSION

The characteristics and genesis of analcite were studied in detail based on the data derived from core description, thin sections identification, X-ray diffraction (XRD), trace element analysis and Field Emission Scanning Electron Microscopy (FE-SEM). XRD results show that the shale samples from the second member of Kong Dian Formation contain dolomite, quartz, feldspar, analcite, clay mineral, pyrite, and calcite, etc., and the analcite content ranges from 1.07 to 59%, with an average of 12.65%. Under microscope, analcite is observed to appear in two types, concentrated laminate type and filling type. The laminated analcite is mainly micritic analcite and interbedded with dolomite and sometimes nodular. The analcite of filling type is mainly distributed in inter- and intra-granular pores as silt to coarse sized cement, and associated with clay minerals, pyrite, and organic matter, with the size of crystal ranging from 2 to10μm.

The Sr /Ba value ranges from 0.32-4.61, with an average of 1.23, and the Pristine/Phytane (Pr/Ph) value ranges from 0.27-1.07, with average of 0.63, which indicates that the sedimentary environment was semi-salty and strong reducing water, which was beneficial to form analcite. The fine-grained shale in this area lacks volcanic detritus, which indicates that the cause of analcite is a non-volcanic precursor. On the basis of the analyses on petrology characteristics, formation temperature (<40℃), and the paragentic relationship with other minerals, the genesis of analcite can be speculated: laminae type analcite is formed through deposition process and sourced from clay minerals, and the analcite of filling type is formed through hydrothermal fluid process. The early precipitation of analcite could inhibit compaction and partially preserve pore spaces. Intergranular pore and dissolved pore of analcite are observed under scanning electron microscope, which increase the ratio of mesopore and macropore.

Overall, the analcite formed in the fine-grain lacustrine shale indicates that the sedimentary environment was moderately saline and alkaline water. The present of analcite enhances the shale brittleness, and improves the physical properties of the reservoir, which helps with analyzing the lacustrine shale oil exploration potential and favorable exploration target selection.