Fig. 4

Petrological and mineralogical characteristics of mudstone and shale. (A): Well FY1, gray-brown mudstone, dense blocky texture, 3173.5 m (core photograph). (B): Well FY1, gray-brown mudstone, 3173.5 m (cross-polarized light); the yellow arrow indicates detrital feldspar, while the clay minerals appear darker. (C): Well FY1, gray-brown mudstone, 3173.5 m (cathodoluminescence); the red arrow indicates carbonate minerals (orange-red), and the blue arrow indicates feldspar (blue). (D): Well FY1, gray-brown mudstone, 3173.5 m (scanning electron microscopy); platy illite (I) and quartz (Q) occupy the pores. (E): Well FYP1, shale, 3456.19 m (core photograph); well-developed laminations are observed. (F): Well LY1, shale, 3692.3 m (cross-polarized light); the yellow arrow indicates carbonate laminations, the red arrow indicates clay minerals, and the blue arrow indicates organic matter laminations. (G): Well FYP1, shale, 3456.19 m (cross-polarized light); the yellow arrow indicates detrital feldspar. (H): Well FYP1, shale, 3456.19 m (cathodoluminescence); the red arrow indicates carbonate minerals (orange-red), and the blue arrow indicates feldspar (blue). (I): Well FYP1, shale, 3456.19 m (scanning electron microscopy); the platy illite–smectite (I/S) interlayer is filled with spheroidal pyrite (Pr).