Sedimentary Stratigraphic Sequence And Petroleum System Of Early Paleozoic Silurian Cratons In Tarim Basin

The hydrocarbon exploration for the cratonic basin has had a history more than one hundred years. And the fact shows that cratonic basin is one of the important hydrocarbon-bearing basins. Up to date, the oil and gas field in quite scale has been found in the basins, such as the Illinois basin, the Gulf Hadson basin, the Tarim basin and so on. And hence, from the basin analysis, dividing the hydrocarbon-bearing system within the basin is an important ring in the course of the exploration. In the paper, the concept of the cratonic basin and the theory of the sequence stratigraphy are used for studying the characteristics of the Early Paleozoic sedimentary formation formed in the Tarim basin being in the cratonic evolution stage. In the meantime, through to trace and correlate the Silurian seismic-geologic horizon on the regional key sections, and carrying out the horizon identifying, stratigraghic correlation and division to the Silurian formation penetrated in the important exploratory wells, the Silurian stratigraghic system respectively has been established in terms of the seismic horizon, geologic level as well as the topping-formation. After that, a series of study about the sequence statigraphy, the depositional facies and the hydrocarbon exploration and so on have been carried out. Finally, the proposed exploratory plan has been offered to the Branch Department of the Tarim Oil Field, and hence, the obvious effectiveness from the exploration for the Silurian has been obtained.Based on the seismic and/or geological horizon from the Silurian to be determined regionally with the unitized criterion, it is recognized that, the reflection from the top and/or bottom boundaries of the Silurian are clear and can be traced and correlated regionally, and that the Silurian is common in the Traim basin. In which, the upper member of the Kepingtake formation, the Tataaiertager formation and the Yimugantawu formation in age of the Silurian distributed mainly in the central area, the middle to upper members of the Kepingtake formation and the Tataairtager formation seen in the northern part, and in the E. Manjiaer area, including the middle to upper members of the Kepingtake formation, the Tataairtager formation and Yimugantawu formation. Moreover, it is seen that, the bituminous sandstones encountered in the well HA-1 and HA-4 that drilled in the northern part of the Tarim basin and that seen in the north slope of the Central part all belong to the upper Kepingtake formation and its middle one consisted mainly of mudstone is southwardonlapping at the north side of the Central uplift. The seismic reflection Tg₄, Tg₄⁰, Tg₄^', Tg₄^" and Tg₅ are respectively corresponding to the top boundaries of the Silurian system, the Tataairtager formation, the upper member of Kepingtake formation (or of the lower sandstone member, or the bituminous sandstone member), the middle member of Kepingtake formation (dark mudstone) and the bottom boundary of the Silurian system, which constructed the seismic-geologic horizon system of the Silurian system.Through the sequence stratigraphic analysis, the Silurian system is divided into four third sequences, from bottom to up are, S_â… , S_â…¡, S_â…¢, S_â…£ Of which, S_â…  corresponding to the lower Silurian series' the middle member of Kepingtake formation, S_â…¡ to the upper member of Kepingtake formation (I, e., bituminous sandstone member), S_â…¢ to the Tataairtager formation and the lower part of the Yimugantawu formation, and S_â…£ to the middle Silurian series' the upper part of Yimugantawu formation. Correspondingly, there are three set of the reservoir-cap rock assemblages to be developed, that is, the upper sub-member of the lower sandstone member combined with the red mudstone, the middle sun-member of the lower sandstone member with the lower sub-member and the Kepingtake formation dark mudstone with the underling sandstone.Based on the lot of well data to be studied as well as the long sections tied with boreholes to be correlated, and combined with the multiple factors, such as the seismic facies, the structural setting, source supply and so on, the depositional system within the Silurian system is analyzed and researched comprehensively. And the horizontal distribution character of the Silurian depositional system has been revealed clearly. The sequence S_â… (the middle member of the Kepingtake formation), has a depositional system consisted of the braided delta-littoral-offshore self facies, the sequence S_â…¡ including one of braided delta-littoral (foreshore, shore face) and seeing tidal flat depositional system locally (in the Central area), both of the sequence S_â…¢ (Tataairtager formation) and the sequence S_â…£ (the Yimugantawu formation), consisted only of a tidal flat depositional system formed mainly due to the tide action.Through the sequence stratigraphic analysis for predicting the play with the stratigraphic-lithologic trap developed within, it is considered that, at an angle of finding the favorable play, for the S. Manjiaer area, should be of the Silurian native reservoir controlled by the stratigraphic-lithologic trap, for the N. Manjiaer area,should be mainly of the truncated -unconformity trap, and for the west and east parts, mainly of the structural trap. The north slope of the Central Tarim and the W. Manjiaer area are the most favorable area for creating the Silurian the oil and gas reservoir in type of the stratigraphic trap. In where, the Silurian formation is commonly onlap southward at the north slope of the Central uplift and created a Silurian stratigraphic onlapping zone. The onlapping zone extends approximately along the east-west direction, beginning the well MN-1 zone at the east and ending the north of the well TZ-45 at the west, with a length up to 300km more. The stratigraphic onlapping zone has been at the slope partition of an ancient uplift since it is formed and without erosion, and hence, where is the most favorable area for finding the native oil and gas reservoir, because where had been in the favorable partition for hydrocarbon accumulation during the two stages of the oil and gas reservoir formation within the Silurian system. In the north part of the Tarim basin, there is a truncated unconformity surface at the top of the Silurian system. The unconformity surface is down dipping to the north due to influence of the area undergone a foreland basin evolution during the Himalayan movement, which formed one best combination with the Silurian strata dipping downward to the south for creating the truncated unconformity traps. For the east part and the west part of the basin, the oil and gas reservoir in type of the structural trap should be as important target.