Yacheng Fm

Hainan-offshore SW. The naming section is located in well Ya13-1-1 in the Yacheng rise, ca. 100 km slightly west of Sanya City, Hainan. Named by Western South Sea Branch, China National Offshore Oil Corp. in 1983.

This formation refers to clastic coal measures below the Lingshui Formation. The formation, lithologically coarse below and fine above, may be divided into two members: Lower member, repeatedly alternating beds of coarse clastic rocks such as purplish gray, thick-bedded conglomerate, sandy conglomerate and sandstone and brownish red, thin-bedded mudstone, ~450 m thick. Upper member, mainly thick-bedded, dark mudstone, with gray or dark gray, multi-bedded sandy conglomerate, sandstone and coal beds, ~460 m thick. The total thickness is 0–900 m. The deposits are thick in the deep part of the basin and thin and even absent on the basin margins. At the seismic section this formation is equivalent to the reflector group between T₇ and T₈.

The upper part of the formation contains some planktic foraminifera such as Globigerina ciperoensis and Globorotalia opima and benthic foraminifera; the calcareous nannofossils include Sphenolithus ciperoensis, Dictyococcites bisectus and Zygrhablithus bijugatus etc.; the sporopollen is represented by the Verrucatosporites-Leiotrilletes-Trilobapollis assemblage. In addition, there are also the dinoflagellates Homotryblium floripes and Hystrichokolpoma rigaudiae etc.

This formation consists dominantly of continental strata with marine intercalations, belonging to littoral-swampy and estuarine deposits. It is the most important source rock in the Qiongdongnan basin.