On Logging-sequence Stratigraphy of Silurian in Tazhong Area, Tarim Basin

Key words: 
• microfacies logging /
• interpretation model /
• logging-sequence Silurian /
• Tazhong area

Abstract: On the basis of core scaling logging data, the logging data interpretation model of Silurian sedimentary micro facies in Tazhong area is established by comprehensive application of conventional logging,natural gamma ray spectrometry(NGS),diplog and image log data.The Silurian in the study area consist of detritus tidal flat deposits,which is divided into subtidal sand flat, tidal channel,intertidal mixed flat and supratidal mudflat.Four logging-sequence boundaries are recongnized.The top and bottom of the Silurian belong to type-I sequence boundary,and its interior belong to type-Ⅱ sequence boundary. Characteristics of these sequence boundaries are described as follows: 1) the pattern in conventional logging and NGS curves is step- sudden change contact, 2) obvious difference either size or direction of dip vectors in the result map of stratum dip vector,3) both stratigraphic attitude and contact relation are also obviously different in FMI images. The Silurian is divided into 3 logging -sequences.which are named as sequenceⅠ,Ⅱ and Ⅲ from the bottom to the top and each one is composed of transgressive system tract(TST)and high stand system tract(HST).TST is mainly sand flat and mixed flat deposits, whose parasequence superimposed pattern shows that the grain size and ratio of sand to mud increase in the vertical, and retrograding bell-shaped series in GR and SP curves.HST is composed of mixed flat and mudflat, whose parasequence superimposed pattern shows that mudrock increases.sand decreases,and ratio of sand to mud decreases in the vertical, which forms accretionary parasequence group,and the straight-ling or box appear in GR and SP curves.By comprehensive analysis of sequence stratigraphy, it is shown that different eustatic rate of relative sea-level result in regular change of accommodation and superimposition in the vertical of microfacies in the transversal of each sequence.By palaeocurrent statistics and analysis from sequence Ⅰ to Ⅲ, it can be seen that current direction in Silurian of this area is double, which is mainly NE-SE.The above analysis shows that logging data is important for the study of sequence stratigraphy,especially for determining stratigraphic series, recognizing sequence boundary,dividing sequence and system tracts and determining palaeocurrent direction.The method is intuitive and clear.