Distribution characteristics and drilling engineering significance of Dongying Formation overflow facies in the gentle slope belt of Huanghekou Sag

Igneous rocks dominated by basalt and tuff of overflow facies are widely developed in the Dongying Formation in the gentle slope belt of Huanghekou Sag. This type of rocks are relatively hard and tend to suffer low rate of penetration (ROP), circulation loss, hole collapse and other problems in the process of drilling, which have greater impact on drilling engineering. In this paper, the volcanic eruption cycles and stages in the first and second Members of Dongying Formation (Ed₁ and Ed₂) were identified and divided by using mud logging and wireline logging data comprehensively. And combined with seismic data, the volcanic overflow facies in BZ34-9 block was described precisely. It is indicated that the volcanic rocks in the study area are mainly distributed in the conjugate strike-slip fault zone of BZ34-9 and KL4-2 structure zones, where fissure type volcanic eruption is dominant. Six eruptive cycles and sixteen eruptive stages can be identified and divided in Ed₁ and Ed₂. The eruptive cycles in the early stage of lower Ed₂ and the early and late stages of Ed₁ are small sized while those in the late stage of lower Ed₂ and the early and late stages of upper Ed₂ are large sized, which reflects the growth process of volcanic activity from weak to strong and then weak gradually. Based on the construction of complex fault and formation model, combined with the simulation of tectonic stress field, geomechanically weak belts or fracture development characteristics can be predicted, which provide reliable basis for the trajectory optimization of development wells and the selection of drilling tools, drilling parameters and drilling fluids.