Understanding mineral composition and depositional mechanisms aids in evaluating gas in place and mechanical properties of shale reservoirs. A method developed to delineate mineral variations and depositional setting combines borehole elemental concentration logs with borehole electrical image logs. Borehole elemental concentration logs provide a continuous measurement of the concentrations of more than 20 elements, which data help in obtaining quantities of mineralogical constituents. Electrical borehole images are used to identify in situ depositional features. Regional mapping of variations of mineral constituents and depositional features indicates sedimentary facies distribution.
The Lower and Upper WuFeng-LongMaxi Formation was studied in 27 wells spanning 100 km west-east across the southern SiChuan basin. From elemental spectroscopy, argillaceous, carbonate, and siliceous lithologies were identified; these were examined by scanning electron microscope (SEM) to investigate their mineralogy and geological origin. Argillaceous minerals were primarily supplied by terrigenous sediments, the majority of carbonate minerals originated from chemical precipitation, and siliceous minerals are associated with siliceous-shell organisms in the Lower WuFeng-LongMaxi strata and terrigenous influx in the Upper LongMaxi strata. A transgressive lag occurring at the base of the WuFeng formation corresponds to carbonate pebbles in cores and bedding-parallel gravels on borehole images. Silty layers deposited by turbidity currents that mainly appear in Upper LongMaxi Formation were readily identified on borehole images.
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