A Study Of Cave Horizons In The Early Hercynian Weathering Crust In The Tarim Basin

The interdisciplinary study between Sequence Stratigraphy and Paleokarst is the frontier trend presently. The main objects for this research are to explore the developmental mechanism, distribution pattern of some paleokarst related with sequence boundary (including huge weathering crust, erosion-type sequences boundary and various sequences boundary paleokarst exposed in short-term) through the establishment of the interrelationship between paleokarst occurred in inner carbonate land and depositional sequences in basin under the cycles of sea level change, then to establish method and techniques for predicting cave systems. It is obvious that this study has prominent theory significant and application value for hydrocarbon exploration in carbonate rocks because the sequence boundary associated paleokarst are the main mechanism of carbonate reservoir of huge subtle traps (e.g. Tahe oilfield, Puguang gasfield). At present, however, the study in this field is just at its initial stage, thus the previous results are very few. In this paper, based on the summarization of previous results, it introduced sequence stratigraphy theory into the study of paleokarst cave horizons in order to establish the interrelation of isochronous beds between the cave horizons occurred in inner carbonate land and the depositional sequences which are corresponding to mentioned cave horizons in time in this basin, then taking isochronous beds as a constraint, conducts a correlation study on cave horizons developed on Tabei, Tazhong and Hetian uplifts across different carbonate lands in this basin to provide theory evidences and method for the study of multiple weathering crust karst cave horizons during the time of intermissive rising of sea level; then linking the hydrodynamic conditions and process of cave horizons and inner cave multicyclic combination of "collapsed breccias interbedded with underflow sediments" with the eustatic sea level change curve in this basin to analyze the karstification ways and characteristics under different geological conditions caused by the eustatic sea level change and then to reveal the origin mechanism of the cave horizons deeply buried. By the research of key methods for predicating the underground cave horizons, a series of methods had been obtained and applied into the predication of cave horizons in Hetian and Tazhong paleouplifts in order to provide the foundations for looking for subtle oil and gas pools in these areas. Main results obtained are as following:1) Has formed the perfect technique for restoring the paleogeomorphy through the integrate research and practices. This technique was applied in the prediction of paleokarst on Hetian, Tazhong and Tabei uplifts. Has concluded early that there are three huge cave horizons (the lower one, C-I ; the middle one, C- II; the upper one C-III) were developed during Early Hercynian;2) Has introduced sequence stratigraphy into the study of paleokarst cave horizons, discussed comprehensively the relationship between the development evolution of cave horizons and the eustatic sea level change, and established the interrelationship between multiple cave horizons and depositional sequences in this basin, and introduced the concept of isochronous bed of cave, the method and principle of the correlation of cave horizons in different region by using isochronous beds;3) Has discussed the forming order of three cave horizons by the evidences of depositional configurations in cave deposits, the intrinsic and extrinsic factors of paleokarst. It concluded that they are developed during the time of intermissive rising of sea level in Early Carboniferous, ach one corresponding to the highstands of sea level change cycles, in an order of the lower the older;4) Has advanced an indirect method for dating the cave horizons through their isochronous beds. Has deduced that the cave horizon C-I , C- II and C-III were individually formed during the deposition of Donghe sandstones, Bioclastic limestones and upper part of Lower mudstones of Lower Carboniferous. Using the Sr isotopic fitting curve drawn up by McArthur (2002) for the dating, the age of the bioclastic limestones (the isochronous bed of C- II) is 358.2 ma;5) Has introduced the concepts of aggradational sequences and aggradational caves, and established the hydrodynamic and geological model of the hysteresis rising effect of underflow surface on the inland caused by abrupt rising of sea level, and pointed out that the inner huge cave multicyclic combination of "collapsed breccias interbedded with underflow sediments" is the effect of the rising of underflow surface delayed to the rising of water table in the inland of basin marginal slope, which is caused by abrupt rising of sea level, and the result of upward persistent erosion of underflow based on the older cave zones; 6) Has concluded that the different development and occurrence of cave horizons on Tabei, Tazhong and Hetian paleo-uplifts were mainly controlled by the differences of their tectonic features and evolutional history. In Early Hercynian, the whole Tabei area was folded and uplifted violently, which led to a basin marginal paleokarst terrane with a large exposure area, homogeneous fractures, well water catchment and high water falling between input and output, as a result, a lots of large scale cave systems with multi cave horizons was developed there. Whereas the Tazhong uplift was uplifted softly in this stage, which led to the small exposure area, poor water catchment, island paleokarst terranes in the center part of basin. As a result, it is in lack of large scale cave systems due to the poor water catchment, but small cave systems may be occurred on the zones of well-developed fractures. The Hetian paleo-uplift is a flat uplift evolved from the separated subsidence of the two limbs in different stage, which led to a large basin marginal karst terrane on a flat slope, poor development of fractures, as a result, 3 cave horizons may be developed owing to that the well water catchments, but multi cave horizons could not be occurred vertically due to the flat slope;7) Using area balancing technique and geometrical relationship, a 2-D model to calculate the pore space of fractures associated with fault development have been developed. The equation created for calculating of fault fracture pore space provides a new way and method for the quantitative description of the development of fault fractures.8) Has established a relatively entire method series for predicating the cave horizons by combine the method of resuming the paleo-physiognomy, description of paleo-fractures , transregional correlation of cave horizons and the locating of cave horizons by the " intersection of fracture zone and water table".9) Has concluded that there is an area of the cave horizon C-I occurred on the Maigaiti slope in the south of Hetian River gas field (northern part of Hetian paleouplift), with a depth of 4500～7500m, occupying an area of 60×80 km~2, could be good carbonate reservoir of subtle traps in this area.