Huagang Fm

Basins offshore NE Zhejiang. The naming section is located in well Longjing No. 2 in the Longjing tectonic belt in the East China Sea shelf basin at 29°52'45″N and 125°56'8″E, and the reference section is in well Tianwaitian No. 1 at 28°32'05″N and 125°00'16″E. Named by the Shanghai Marine Geological Party, former Ministry of Geology and Mineral Resources in 1982.

Located above the Pinghu Formation and below the Hailongjing Formation, this formation is a sequence of grayish white sandstone with dark gray mudstone and coal, the middle and upper parts of which are intercalated with variegated and brownish red mudstone. Lithologically it may be divided into two members. Lower member is marked by grayish white sandstone and sandy conglomerate with dark gray mudstone in the lower part, and alternating beds of gray and dark gray mudstone and grayish white sandstone in the upper part; this member is intercalated with small amounts of bituminous coal and bands. Upper member consists of grayish white sandstone and sandy conglomerate with dark gray mudstone in the lower part, and alternating beds of gray, greenish gray, variegated and brownish red mudstone and light gray sandstone with minor coal in the upper part. Its thickness and lithology show relatively wide lateral changes. The Baochu slope is relatively fine in grain size and thin; eastward to the Changyuan area the lithology becomes coarse and massive sandstone is well developed; in the Longjing area north of Changyuan the lower part of the formation is thick and coarse-grained and massive sandstone is well developed, with the lower part of the formation being missing to different degrees in well Pinghu 1 and well Pinghu 2.

The Huagang Formation in the well columnar sections commonly contains abundant sporopollen, represented by the Quercoidites-Trilobapollis-Pinuspollenites and Abietineaepollenites-Magnastriatites assemblages. The charophytes found in the Huagang Formation in 7 exploratory wells include the Krassavinella lagenalis-Nitellopsis Helvetica assemblage; the bivalve Taiwancorbicula parca is found at depths of 3141–3148 m of well Yuquan No. 1. In addition, resedimentary communities of marine fossils such as late Eocene Dinophyceae and calcareous nannofossils are also found in the formation of 8 exploratory wells.

The Huagang Formation belongs to fluvial-deltaic deposits.