Fine Logging Evaluation Of Volcanic Rock In Continental Scientific Drilling Borehole In Songliao Basin:Method Research And Practice

Humanity is encountering two major problems,resource shortage and global climate warming.Songliao basin is rich in natural resources.During Cretaceous,the Songliao basin experienced typical“greenhouse climate”which are affected by volcanic activities.The borehole SK-2 East in Songliao basin of China Cretaceous Continental Scientific Drilling Project,funded by ICDP,is drilled and continuous,abundant and complete geophysical logs is acquired.Fine logging evaluation is carried out by comprehensive study of geophysical logs.Lithological profile,petrophysical parameters and pore structure characteristics were obtained.The results support for the new breakthrough of resource exploration and the revelation of Cretaceous volcanic activityThe response characteristics analysis of volcanic rocks of Yingcheng formation in SK-2 east borehole shows that physical properties revealed by geophysical logs vary with lithology,especially for natural gamma ray?GR?,density?DEN?and resistivity?Rt?.The radioactivity and electric conductivity of tuffs are the strongest.Agglomeratic lavas has the low density because of high porosity.The density of rhyolites is the highest and the radioactivity is the weakest.The fluidal structure of rhyolites,the agglomeratic texture of agglomeratic lavas and the tuff texture of tufflavas are observed clearly in microresistivity images.The differences in element and oxide contents among lithology are mainly reflected in the content of Fe,SiO₂ and K₂O+Na₂O.There are obvious difference in petrophysical characteristics of volcanic lava and volcaniclastic rock.According to the lithology profile identified by using cross plot technique and particular logging method,cluster analysis was launched with the content of quartz+feldspar and dark mineral based on the first order and second order classification of volcanic rocks.The accuracy of identification of volcanic rocks in Songliao basin was improved.Total porosity,effective porosity,fracture porosity and primary porosity were calculated by using multi-mineral model,empirical formula and new interpretation model.The results show that the total porosity of agglomeratic lava is greater and up to 17%.The total porosity range of tufflava changes greatly between 3%and 15%.The total porosity of rhyolite varies from 7%to 14%.Fractures are well-developed in tufflava.The porosity range and the heterogeneity of volcanic rocks in SK-2 east borehole increases with depth.Both rhyolite and tufflava have the better pore structure which are favorable for exploring resource and performing long-term observations and fluid experiments.The secondary porosity can cause the change of porosity spectrum.But the increase of right proportion of spectrum can't completely represent the development of secondary porosity.The characteristics of porosity spectrum in the transitional areas of the volcanic sequence can be used to indicate genesis and evolution of magma.