Liushagang Fm

Beibu Gulf region. The naming section is located in Maican No. 2 well at Xilian, Xuwen County, Guangdong. Named by Maoming Petroleum Company in 1975.

The Liushagang Formation refers to gray clastic rocks between the Changliu Formation and Weizhou Formation and takes the 2120–3227 m interval of Maican No. 2 well as the stratotype. It is marked by alternating non-isopachous beds of dark gray, grayish black and grayish green mudstone-shale and grayish white, medium and fine sandstone. Three members are distinguished: Lower member has lower part of alternating beds of brownish red mudstone and grayish white pebbly sandstone and the upper part of dark gray mudstone and grayish white and grayish brown oil-bearing sandstone and sandy conglomerate. Middle member, thick-bedded, dark gray mudstone-shale, pebbly sandstone and oil-bearing pebbly sandstones, locally with black basalt. Upper member, alternating beds of dark gray mudstone-shale and light gray siltstone and sandy conglomerate; the top is separated from the Weizhou Fm by brownish red mudstone. The thickness is 1107 m. The lithology is less varied and the thickness varies greatly: from the basin center toward margins the rocks become coarse and thin, the thickness ranging from 32 to 2000 m. In the seismic profile, the formation is equivalent to the reflector group between reflectors T₄ and T₇, of which reflectors T₅ and T₆ are approximately equivalent to the Second Member (middle member) and Third Member (lower member) of the formation.

In ascending order the sporopollen and algae are divided into four assemblages: 1）the Monocolpopollenites - Crassoretitriletes assemblage; 2）the Salixipollenites – Momipites – Operculumpollis assemblage; 3）the Quercoidites – Ulmipollenites – Alnipollenites assemblage; and 4）the Leiosphaeridia - Granodiscus assemblage. Ostracods are mainly the genus Sinocypris, mainly including the species S. excelsa and S. funingensis etc. Its age is Eocene to early Oligocene.

This formation is freshwater lake deposits and the most important oil source bed in the Beibu Gulf basin.