| Since the 1980s, sequence stratigraphy quickly became the focus of geology due to the rigorous scientific, logical and effective forecasting capabilities. Compared with the marine sequence, terrigenous sequence has lots of characteristics, such as abundant sediments, complex regional tectonic events, and various sedimentary facies. In this study, the principles of sequence stratigraphy will be used to identify the main factor and evolution rule of the continental rift basin, to predict the favorable reservoir and lithologic trap, and improve the petroleum exploration and development of continental rift basin in China.The sequence stratigraphic framework and model of episodic continental rift basin is built based on the basement subsidence history in this study. With the application of this method, the sequence stratigraphic framework and sedimentary facies evolution is built in Artala sag of Erlian basin, and the favorable reservoir facies belt is located.The continental rift basin, controlled by the alternation of accommodation which is caused by the basement subsidence, is different from the passive continental margin basin. Therefore, the continental rift events are considered during establishing the sequence stratigraphic framework in this study. The first order sequence, equivelance to the geotectonic movement surface, is generation stratigraphic unit. The second order sequence, corresponding to the regional tectonism unconformity, is prototype formation unit. According to the subsidence stage, each basin prototype can be devided into four subside filling units, that is PSU(preliminary subside filling unit), SSU(speediness subside filling unit), DSU (decelerate subside filling unit) and ISU (inverse subside filling unit). All of three filling units could be comprised by one or more chasmic-reverse cycles which is called the third order sequence. The third order sequence of the PSU is mainly comprised by prograding or accretional LST and HST because of the smaller subsidence area, undulating terrain and lower subsidence rate which cause the accommodation increasing rate is much smaller than the sedimentary supply ratio. While, the third order sequence of the SSU, which suffer from the intensive cracking movement and extensive lake basin areas, is mainly comprised by the retrograding TST, with thinner LST that is locally developed in gentle slope belt slope break and bordering fault zone, HST is developed at the finally stage of structural movement that is constituted by thinner coarsening upward sequence. Overall, the accommodation increasing rate is much faster than the sedimentary supply ratio at that time. The third order sequence of the DSU is characterized by the fact that the accommodation increasing rate is smaller than the sedimentary supply ratio due to the stress release and reverse.This sequence is mainly include LST and HST, with thinner TST. The third order sequence of inverse subside filling unit is constituted by LST, with mainly coarse fluvial and alluvial plain facies sedimentary, as a result of the accommodation increasing rate is much smaller than the sedimentary supply ratio which is controlled by the regional stress reversal, suspended basement subsidence and even basement uplift. Lately, the basin is eroded because of the basement uplift, the sedimentary of LST is eroded and remove out of the basin.The above sequence stratigraphy model of continental rift basin has been untiled in Aer sag, Erlian basin. The formation in Aer sag is divided into the Paleozoic first-order sequence and Mesozoic-Cenozoic first-order sequence. The Mesozoic-Cenozoic first-order sequence can then be divided into Jurassic basin prototype (second-order sequence) and Lower Cretaceous basin prototype. According to the seismic local unconformity and abrupt contact on well log, the Lower Cretaceous second-order sequence can be divided into six three-order sequences(rift-inversion cycle), that is K1ba3 sequence, K1ba4 sequence, lower K1bt1 sequence, upper K1b1 sequence, K1b2 sequence and Saihantala sequence. Combining the tectonic subsidence rate, these six sequence can be grouped into three kind, that is preliminary subside filling unit (K1ba3 sequence), speediness subside filling unit (K1ba4 sequence, lower Klbt1 sequence, upper K1bt1 sequence), decelerate subside filling unit (K1bt2 sequence and Saihantala sequence). There is no classic inverse subside filling unit because Aer sag is in the middle of NE AsiaThe similarity attribute has been used to discover the distribution of fault for each sequence. In K1ba3 sequence, the activity of border fault is weak and the sequence is controlled by fault in the sag. The sequence is bounded by the faults around well aer6 and faults around well aer61 which have opposite dip. In K1ba4 sequence, the border fault becomes active, and the subsidence center is at the foot of border fault. Just like the situation in K1ba3 sequence, there is a republic fault combination with NE and nearly N strike. According to the bisector of acute angle of the strike, the stress which results in the rift of Aer sag may be NNE-SSE. In lower K1bt1 sequence, the subsidence rate is fast and the tectonic is so active that three secondary faults on the steep slop are formed. At the same time, a fault combination with strikes of NW and NNW which is almost perpendicular to the border fault has been discovered. These faults are assumed to be related to the tectonic inversion which may be the extrusion stress from NNW according to the bisector of acute angle of the strike. This tectonic inversion may be the motion for the decrease of subsidence rate and uplift at the end of each sequence. It is seemed that there is no such a sign of fault combination in K1ba4 sequence and K1ba3 sequence, which signify that the tectonic inversion in these two sequences is weak. In upper K1bt1 sequence, most of the fault is inherited from the fault in lower K1bt1 sequence. The differences is the middle secondary fault separate into two segments among which forms an accommodation zone between secondary faults. The fault with NW strike which may be created by tectonic inversion develops in upper K1bt1 sequence. These faults have a shape of "Y" with NE extension. Because "Y" shape faults are the adjustment of extension, these faults may be the results of the NNW tectonic inversion compression stress.Controlled by the background of subsidence rate, each sequence has different characteristic in system tract. In K1ba3 sequence, the subsidence rate is very low, so the creation rate of new accommodation is smaller than the sediment supply. LST and HST which is mainly in aggradation and progradation account for most of the sequence, that is 20% and 51%, respectively, while TST only accounts for 29%. In K1ba4 sequence, the subsidence rate is fast and the rate of accommodation frequently outpace the sediment supply. TST is the dominate composition in this sequence, with 60% of the thickness of the formation, while LST and HST only account for 20% and 20% respectively. In lower K1bt1 sequence, the subsidence rate is fast and the rate of accommodation frequently outpace the sediment supply. TST is the dominate composition in this sequence, with 49% of the thickness of the formation, while LST and HST only account for 26% and 25% respectively. In upper K1bt1 sequence, the subsidence rate decreases and the LST and HST will dominate. TST is still the main composition in this sequence, with 40% of the thickness of the formation, while LST and HST only account for 35% and 25% respectively. As to the K1bt2 sequence and Saihantala sequence which belong to Decelerate Subside Filling Unit, LST and HST may become the main ingredient and TST maybe very thin. Because these two sequences are not the favorable target in petroleum exploration, it will not be discussed specifically in this thesis.Basin System.According to sequence and sedimentary facies analysis on single well, the sedimentary facies distribution and evolution has been researched. During the deposition of K1ba3 sequence, the deposition is limited in the middle of research area. Sediment is mainly supplied by the fan delta in the east of research area during the LST. When it comes to TST, the lake expands and most of the sediment comes from the fan delta while the other is from the braid river delta on the slope. During HST, the progradation is well developed especially in the northeast of the research area, there maybe a progradation delta. During K1ba4, the subsidence area become bigger, but the middle and west of the research area may still be in shallow water or exposed. During LST, the sediment area is limited; most of the sediment is from the fan delta on the steep slop in the middle of research area around well aerl6. During TST, the deposition center moves to the east of research area around well aerl. The terrain steepens, and formation continued onlap toward northward and westward, which indicates the rapid expansion of the lake. Provenance is mainly from a retrograding fan-delta near well aerl in the accommodation zone. The period of HST is short; resulting in the deposition of HST is thin. Sediments are mainly from the fan delta on the east accommodation zone around well aerl and braid river delta around well ad2 and another braid river delta around well aer26 on the west accommodation zone. During the development of lower K1bt1 the tectonic is active. The secondary fault makes the subsidence center move to the center of depression. During LST, sediment is mainly from the fan delta around well aer21 and braid river delta on the west accommodation zone, the other is from fan delta around well aer11 on the east accommodation zone and fan delta around well aer4 on the middle secondary fault. During TST, lake expands. The sediment mainly comes from the west accommodation zone, then from the braid river delta around well ad2 on the east accommodation zone and fan delta on middle secondary fault. During HST, sediment is mainly from the fan delta around well aer2 on the east accommodation zone, then from the east accommodation zone and fan delta around well aer4 in the middle secondary fault. The fan deltas around well aer15 and well aer4 have obvious progradation compared to the TST below. During the formation of upper K1bt1, the tectonic subsidence rate decreased but still is fast. The TST is the predominant composition. During LST, sediment is mainly from the fan delta around well aer15 on the east accommodation zone and braid river delta around well aer26 on the west accommodation zone. The less important provenance is from the fan delta around well aer21 on the west accommodation zone, the sublacustrine fan around well aer41 and aer6. During TST, sediment is mainly from the fan delta around well aer15 on the east accommodation zone, and the rest are covered by lacustrine mud. During HST, the progradation develop well.Sediment is mainly from the fan delta around well aerl5 on the east accommodation zone and braid river delta around well aer23 on the east accommodation zone.The favor reservoir facies has been analyzed under the framework of sequence stratigraphy. In K1ba3 sequence, the most favorable target is the HST sand which is located around well aer6 on the foot of antithetic fault. In K1ba4 sequence, the most favorable reservoir is HST sand in the foot of antithetic fault in the accommodation zone. In lower K1bt1 sequence, the most favorable reservoir is LST, TST and HST. In LST, the favorable target is the slum fan in front of fan delta around well aerl on the Hada anticline and the sublacustrine fan around well aer3-1 and aer41. In TST, the target is in the tectonic high in west accommodation zone and the sublacustrine fan around well aer3-1 and aer41. In HST, the favorable target is the tectonic high which is 2000m away from well aer2 and the fan delta around well aer3 which is bounded by the antithetic fault and secondary fault. In Upper K1b1 sequence, the whole system tract is the favor target. In LST, the favorable facies is the sublacustrine fan in the center of depression, the near-shore subaqueous fan around well aer3 and its extension part into the lake center, and the front of braid river delta around well aer26. In TST, the favorable facies is the mouth bar in the front of fan delta in Hada anticline and sublacustrine fan in the northwest of well aer41. In HST, the favorable facies lies in the tectonic high in Shamai anticline and sublacustrine fan in the center of depression.The innovation lies in:According to the intensity of rift activity, a prototype formation unit can be divided in to four subsidence filling units. Each subsidence filling unit corresponds to one or several rift-Reverse cycle (three sequences). Basement subsidence rate not only control the evolution of the third-order division and internal systems tracts, and even control the formation of parasequences and parasequence group. Filling established sequence stratigraphic model of continental rift basins based on the above understanding. And applying the model to instances of depression among Al Erlian Basin, and to explore the ancient geomorphology and sedimentary Al depression sequence stratigraphic framework under the constraint of phase evolution, predict the distribution of favorable reservoir facies belts. |
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