Biostratigraphy and reservoir characteristics of the Ordovician Wufeng Formation-Silurian Longmaxi Formation shale in the Sichuan Basin and its surrounding areas, China

WANG Hongyan,SHI Zhensheng,SUN Shasha

Table 1 Lithofacies assemblages and logging facies of graptolite zones in the Wufeng-Longmaxi Formation of Sichuan Basin and its surrounding areas.

Graptolite zone Stratigraphy Lithologic composition Thickness/m Characteristics of bottom interface Characteristics on logging curve Sedimentary cycle Formation Member Submember Sublayer Gamma ray (GR) Acoustic time (AC) Resistivity N2 Liangshan Formation/ Shiniulan Formation Gray marl, biogenic limestone intercalated with calcareous shale Angle unconformity, abrupt change of lithology and lithofacies, strong amplitude, high continuous peak reflection, abrupt decrease of GR and AC, abrupt increase of resistivity LM9/ N1 Longmaxi Formation Long-2 Dark gray limestone, greenish gray mudstone, silty mudstone 48.0- 446.5 Conformity, AC and GR are funnel-shaped under the interface, and are box- shaped above the interface LST LM6- LM8 Long-1 Long-12 Dark gray, grayish black mudstone, shale 9.9- 110 Conformity, the shape of the logging curve changes from funnel shape to box shape under the interface HST LM5 Long-11 Long- 114 Silty shale, calcareous shale 4.3- 19.7 Conformity, the shape of the logging curve changes from box shape to funnel shape under the interface Funnel shape Funnel shape Funnel shape TST LM4 Long- 113 Argillaceous shale, carbonaceous shale 2.1- 14.5 Conformity, the shape of the logging curve changes from funnel shape to box shape under the interface Box shape Box shape Box shape LM2- LM3 Long- 112 Carbonaceous shale, argillaceous shale 2.0- 9.2 Logging curve changes from finger or box shape into funnel shape Funnel shape Funnel shape Toothed funnel shape LM1 Long- 111 Siliceous shale, rich in graptolite 1.3- 4.2 Light gray limestone abruptly changes to siliceous shale, sudden rise of GR and AC, and fall of resistivity High value and finger shape Box shape Box shape WF4 Wufeng Formation Guanyinqiao Member Light gray mudstone, argillaceous limestone, marl, limestone, rich in shell fossils, brachiopods and trilobites are most abundant 0.2- 1.0 Black shale rich in graptolite abruptly changes to gray limestone or marl, abrupt fall of GR and AC, and abrupt rise of resistivity Low value and finger shape Low value and finger shape High value and finger shape HST WF2- WF3 Graptolite shale section Black shale, rich in graptolite and high in carbonate content 0.5- 14.5 Parallel unconformity, the interface shows strong amplitude and high continuous peak reflection, abrupt rise of GR and AC above the interface, and abrupt fall of resistivity High value and box shape High value and box shape Low value and box shape TST Baota Formation Light gray nodular limestone Low value and box shape Low value and box shape Low value and box shape