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Natural Gas Industry B 2 (2015) 421-429
Research article
www.elsevier.com/locate/ngib
Sequence stratigraphie features of the Middle Permian Maokou Formation in the Sichuan Basin and their controls on source rocks and reservoirs*
Su Wang Jiang Qingchun , Chen Zhiyong , Wang Zecheng , Jiang Hua , Bian Congsheng ,
Feng Qingfua, Wu Yulinb
a PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China b Geophysical Exploration Company, CNPC Chuanqing Drilling Engineering Limited Company, Chengdu, Sichuan 610210, China
Received 18 March 2015; accepted 8 September 2015 Available online 2 March 2016
Abstract
Well Shuangyushi 1 and Well Nanchong l deployed in the NW and central Sichuan Basin have obtained a high-yield industrial gas flow in the dolomite and karst reservoirs of the Middle Permian Maokou Formation, showing good exploration prospects of the Maokou Formation. In order to identify the sequence stratigraphic features of the Maokou Formation, its sequence stratigraphy was divided and a unified sequence strati-graphic framework applicable for the entire basin was established to analyze the stratigraphic denudation features within the sequence framework by using the spectral curve trend attribute analysis, together with drilling and outcrop data. On this basis, the controls of sequence on source rocks and reservoirs were analyzed. In particular, the Maokou Formation was divided into two third-order sequences — SQ1 and SQ2. SQ1 was composed of members Mao 1 Member and Mao 3, while SQ2 was composed of Mao 4 Member. Sequence stratigraphic correlation indicated that the Maokou Formation within the basin had experienced erosion to varying extent, forming "three intense and two weak" denuded regions, among which, the upper part of SQ2 was slightly denuded in the two weak denuded regions (SW Sichuan Basin and locally Eastern Sichuan Basin), while SQ2 was denuded out in the three intense denuded regions (Southern Sichuan Basin—Central Sichuan Basin, NE and NW Sichuan Basin). The development of source rocks and reservoirs within sequence stratigraphic framework was significantly affected by sequence boundary; the grain banks that can form effective reservoir were predominately distributed in SQ1 highstand systems tract (HST), while effective source rocks were predominately distributed in SQ1 transgressive system tract (TST). It is concluded that the sequence division method is objective and reasonable, which can effectively guide oil and gas exploration in this region.
© 2016 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Sichuan Basin; Middle Permian; Spectrum trend attribute analysis; Sequence stratigraphy; Denuded region; Source rocks; Reservoirs; Grain bank
Natural gas exploration of the Permian Maokou Formation, Sichuan Basin has lasted for more than half a century [1]. Recently, Well Shuangyushi 1 and Well Nanchong l deployed in the NW and central Sichuan Basin obtained high-yield
* Fund project: Special and Significant Project of National Science and Technology "Study on oil and gas resources potential of marine carbonate rocks and formation conditions and distribution of giant oil and gas fields" (No. 2011ZX05004-001).
* Corresponding author.
E-mail address: suwang91@163.com (Su W.).
Peer review under responsibility of Sichuan Petroleum Administration.
industrial gas flow in the dolomite and karst reservoirs of the Maokou Formation, revealing that the Maokou Formation has good exploration prospect in the peripheral region of southern Sichuan Basin. Therefore, the Maokou Formation has become a focus for study and a lot of effort has been put into the study on its sedimentary facies, karst palaeogeomorphology, and dolomite reservoir genesis. Sequence stratigraphic analysis, as an effective method for reservoir prediction, is naturally one of the most important content in study [2—6]. However, since the Maokou Formation has experienced denudation at late stage, and traditional sequence division scheme is significantly influenced by subjective human factors, its sequence boundary
http://dx.doi.org/10.1016/j.ngib.2015.09.018
2352-8540/© 2016 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
is controversial and poorly correlated, unable to be used to effectively predict the distribution of source rocks and reservoirs, and plays a limited role in guiding exploration and production. In order to minimize the impact of human factors, make sequence division scheme more objective, and highlight the prediction of sequence stratigraphy on source rocks and reservoirs, the sequence of the Maokou Formation was divided based on INPEFA sequence analysis technique, coupled with outcrop and drilling data in this paper. The results indicate the source rocks and reservoirs within sequence stratigraphic framework follow some apparent patterns in vertical direction.
1. Geologic overview
In late Carboniferous, the Yunnan Movement happened in the Sichuan Basin [2,7,8], leading to successive deposition of the Middle Permian Liangshan, Qixia and Maokou formations under a peneplain geomorphologic background. Particularly, the Maokou Formation is a set of marine carbonate deposits being 150-400 m thick (Fig. 1), dominated by gray, dark gray micrite, micrite or sparite bioclastic limestone interbeded with marl and shale, bounty chert nodules or bands, and dolomite in local areas. It is divided into four lithological intervals from bottom to top based on the lithological, electrical and biological fossil characteristics: © Mao 1 Member (P2m:) is composed of dark gray, gray black micrite interbeded with marlstone, with high shale content; © Mao 2 Member (P2m2) is dominated by taupe micritic bioclastic limestone, with sparite bioclastic limestone in local areas, and chert nodules or chert bands; © Mao 3 Member (P2m3) is dominated by light gray calcsparite bioclastic limestone; © Mao 4 Member (P2m4) is made up of dark gray, gray black bioclastic micrite
and micrite bioclastic limestone [7,8]. In the late Middle Permian, affected by the Dongwu Movement, the Maokou Formation was eroded to varying degrees in the whole basin [7-10]; as a result, Mao 4 Member was left only in the local area, giving rise to a regional unconformity between Middle and Upper Permian strata.
2. Sequence stratigraphic division
2.1. Technical principle of spectral trend attribute and sequence division
Spectral trend attribute analysis technique is a stratigraphic cycle identification technique by making use of the spectral analysis of well logs [11]. With cycle stratigraphy as theoretical basis, this technique converts well logs into one INPEFA (Integrated Prediction Error Filter Analysis) curve by introducing modern digital signal processing means, to make the characteristics of stratigraphic cycle hidden in wells more pronounced [12](Fig. 2), which is helpful for the analysis on sequence stratigraphy and sedimentary cycles. In China, this method has been applied in the clastic rocks in eastern China [13], but it is rarely applied in marine carbonate rocks [12].
Previous studies show that the Maokou Formation in the Sichuan Basin is dominated by carbonate deposits of open platform facies, hence, LST is absent in the third-order sequence of the Maokou Formation [2]. But the significant changes in eustatic sea level in the Maokou Period resulted in wide changes in its shale content, and the GR curve amplitude with obvious changes can better reflect the features of sedimentary cycles. Furthermore, since the GR curve is slightly affected by borehole conditions and is basically included in
Fig. 1. Isopach map of the Middle Permian Maokou Formation, Sichuan Basin.
Fig. 2. Flow chart of sequence division using INPEFA technique and its schematic model.
the logging suites of existing wells in this area, the GR curve was selected to obtain the INPEFA curve through conversion, which is then used to divide sequence stratigraphy.
Firstly, MESA (Maximum Entropy Spectral Analysis) was performed on the GR curve in order to obtain the MESA_GR curve; then, PEFA (Prediction Error Filter Analysis) was performed in order to obtain the numerical error between the MESA predicted value and the true value of well logs at corresponding depth points, namely PEFA_GR curve, at this time, error value is the result of actual data value minus filtered value. The PEFA_GR curve was an irregular dentate curve varying along the vertical line. This curve can be taken as an indicator to interpret stratigraphic continuity: Negative peaks (large negative error) represent possible flooding surfaces, while positive peaks (large positive error) represent possible sequence boundaries; different peaks (different errors) indicate isochronous surfaces of different sequence orders [11,14,15]. Analysis on the cycles of the Maokou Formation reveals that the PEFA curve obtained from the GR curve after filtering processing can get better application effect in this area. Finally, integrating processing was performed of the PEFA-GR curve [11] to obtain the INPEFA-GR curve that could be used for sequence division based on its positive and negative trend and their turning points (Fig. 2).
In marine carbonate sedimentary environment, for the INPEFA-GR curve converted from the GR curve, an overall
negative trend (INPEFA curve value gradually decreases from bottom to top) indicates that the shale content gradually decreases from bottom to top and the climate gradually become arid, which represents a regressive sedimentary sequence; whereas an overall positive trend (INPEFA curve value gradually increases from bottom to top) indicates the shale content gradually increases from bottom to top, which represents a transgressive sedimentary sequence [11,14]. A turning point of the curve represents a possible sequence boundary or a characteristic interface of internal sequence. A negative turning point of an interval indicates the maximum shale content and the minimum depositional rate of the interval, which represents a possible flooding surface; whereas a positive turning point indicates the minimum shale content of a certain well section, which represents a possible sequence boundary [11,14]. Different levels of trends and turning points correspond to equivalent levels of sequence structures and sequence boundaries. The conversion of the GR curve of the Maokou Formation reveals the INPEFA_GR curve of the Maokou Formation has two positive trends and two negative trends together with two negative turning points and two positive turning points. Each turning point corresponds to one peak in PEFA_GR curve at corresponding depth (Fig. 3). Based on the positive and negative trends and turning points of INPEFA_GR curve, coupled with the comprehensive analysis on outcrops, core thin sections (Fig. 4) and the development time of third-
Fig. 3. Schematic map of sequence stratigraphic division in the Maokou Formation, Sichuan Basin (Well Zhougong 1).
order sequence, the Maokou Formation in the Sichuan Basin is divided into two third-order sequences, SQ1 and SQ2 (Fig. 3).
2.2. Sequence development features of the Maokou Formation
Based on the spectral trend attribute analysis of the GR curve in a typical single well, spectral trend attribute analysis was performed on the GR curves of 117 wells. Then, the third-order sequence stratigraphic framework of the Middle Permian Maokou Formation in the Sichuan Basin was established by referring to the 18 sequence stratigraphic framework profiles throughout the entire region. Through comparison, it is found that the spectral trend attribute of GR curve can well reflect the variation in the sedimentary cycles of the Maokou Formation in the entire region, with the boundaries of the two third-order sequences and their internal constitutive characteristics shown clearly.
2.2.1. Features of sequence boundary
Based on the turning points of the INPEFA curve, combined with outcrop data, the sequence boundaries, SB1-SB3 have
been identified. In particular, SB1 and SB2 are lithology and lithofacies transitional surfaces (Type II) and SB3 is an uplifting erosion unconformity surface (Type I). Sequence boundary SB1 is between the Qixia Formation and Maokou Formation, and the sedimentary features above and below it are dramatically different. Qi 2 Member below SB1 is composed of light gray, gray thick bioclastic limestone, dolomite or leopard spot dolomitic limestone; Mao 1 Member above SB1 is composed of dark gray, gray black thin-medium augen limestone interbeded with gray black thin marl (Fig. 4a and b), with high shale content. On well logs (Fig. 3), upper Mao 1 Member and lower Qi 2 Member are in abrupt contact; the PEFA curve shows apparent positive peaks; the INPEFA curve is close to zero; the magnitude color is yellow to red, representing a positive turning point from a negative trend to a positive one.
Sequence boundary SB2 corresponds to the top surface of Mao 3 Member, above it are high energy sparite bioclastic limestone and sparite algal limestone (Fig. 4e), whereas, below it are low energy bioclastic micrite, micrite and argillaceous micrite, showing obvious variations in lithology and
Fig. 4. Plate of outcrop and core thin section of the Maokou Formation, Sichuan Basin. Note: a. Sequence boundary SB1. Below it is the light gray thick massive dolomite or leopard spot dolomitic limestone of Member 2 of Qixia Formation and above it is the TST deposits of SQ1 composed of dark gray-gray black thin-medium eyeball-shaped limestone interbeded with marl of Mao1 Member with thickness decreasing and the color deepening upwards from the section of Guangyuan Changjianggou; b. Maximum flooding surface MFS1 of SQ1, gray black—black gray argillaceous micrite and marl. The TST of lower part is composed of gray, gray black medium-thick eyeball-shaped limestone, interbeded with marl, with color deepening and individual layer thickness increasing upward, while the HST of upper part is composed of thick light gray pseudo-eyeball-shaped bioclastic limestone, with color deepening and individual layer thickness increasing upward, it was from the section of Guangyuan Changjianggou; c. Sequence boundary SB3 with weathering clay layer. Below it is the HST deposits of SQ2 dominated by light gray bioclastic limestone and above it is the Emeishan basalt. It was from the section of Emei Longmen; d. "Eyelid limestone" in the TST of SQ1 (TST1) with high shale content, and bioclast in orientation arrangement, from 2718.14 m of Well Luobei 4; e. Sparite bioclastic limestone of the HSTof SQ1 (HST2), with a strong cementation, from 3190.13 m of Well Bao 34; f. Sparite bioclastic limestone of the HST of SQ1 (HST2), with dissolved pores filled with bitumen, from 1675.30 m of Well Weiyang 17 (Zoom in 25 times).
lithofacies. SB2 has prominent logging response features, since its INPEFA curve is similar to that of SB1 (Fig. 3) and its PEFA curve shows obvious positive peak. The positive peak of PEFA curve converted from GR curve after the median filtering of varying lengths (3 m, 6 m, 9 m, etc.) still exists, indicating that SB2 is a reliable sequence boundary.
Sequence boundary SB3, the top surface of SQ2 (the top surface of SQ1 in intensively denudated region), is between the Maokou Formation and its overlying the Longtan Formation or Wujiaping Formation (Emeishan basalt above the Maokou Formation in the SW Sichuan Basin) (Fig. 4c). The Dongwu Movement happened in late Permian Maokou Period resulted in the overall uplifting of the region and the top of the Maokou Formation erosion to varying degrees, thereby, it is an uplifting erosion unconformity surface, belonging to Type I sequence boundary.
2.2.2. Sequence internal constitutive features
Sequence SQ1 includes sections from Mao 1 Member to Mao 3 Member, in which transgressive system tract (TST) generally appears in Mao 1 Member and the c submember of Mao 2 Member; highstand system tract (HST) generally appears at the top boundary of Mao 3 Member; the maximum flooding surface (MFS) corresponds to the top boundary of the c submember of Mao 2 Member, which is composed of gray black argillaceous bioclastic micrite and marl (Fig. 4b), with the highest shale content; it has prominent well logging
response features (Fig. 3), since 1) GR and AC curves rapidly decrease, showing "scarp" feature; 2) resistivity curve manifests an obviously low value; 3) PEFA curve shows negative peak; 4) INPEFA curve is close to 1; 5) its magnitude color is green to blue, representing a negative turning point from positive trend to negative trend. TST is dominated by dark gray, gray black bioclastic micrite and argillaceous micrite, with marl in local area, and is deepening in color, thinning in individual layer and increasing in shale content from bottom to top (Fig. 4a and b). The outcrops are characterized by "eyeball-shaped limestone", chert bands or tuberculosis in local area, and rich Cryptospirifer brachiopod fauna. It presents high amplitude on GR curve, concave-shaped low value on resistivity log, and an apparent positive trend on INPEFA. HST was dominated by taupe micrite bioclastic limestone and sparite bioclastic limestone, with color lightening, thickness increasing and shale content decreasing from bottom to top. At the top of HST, namely Mao 3 Member, the lithology changes into light gray thick sparite bioclastic limestone and dolomite, with medium—low value on GR curve. To the top of HST, GR value reduces to the minimum, and resistivity value increases significantly and INPEFA also manifests obvious negative trend, which proves the gradual drop of shale content upward.
Sequence SQ2 composed of Mao 4 Member, only remains in local area in the basin due to denudation. It is dominated by gray, dark gray bioclastic micrite and green algae micritic
limestone, with higher shale content than the underlying Mao
3 Member. It shows medium-high amplitude on the GR curve and medium—low value on resistivity curve. The maximum flooding surface (MFS) is difficult to identify by using outcrop, logging and seismic data, however, the TST and HST can be easily identified by using PEFA and INPEFA curves (Fig. 3), indicating the advantages of this technique. The PEFA curve shows negative peak, and the INPEFA curve shows negative turning point, both of which show more prominent features after the segment analysis of individual sequences. The magnitude color is green to blue green.
Relatively speaking, the PEFA of TST shows dentate shape (the INPEFA curve rises in dentate shape), while HST manifests relatively flat vertical line (the INPEFA curve drops as a straight line, Fig. 3) as a result of the high shale content of TST and its significant variation in vertical direction, which could also be proved by the shape of GR curve (in dentate shape); HST is composed of pure limestone, with shale content gradually decreasing from bottom to top. This feature is universal in the entire region and can be taken as a reference for sequence and system tract division.
3. Stratigraphic denudation within sequence framework
Within sequence stratigraphic framework, analyzing the denudation features of the Maokou Formation based on the distribution and preservation degree of SQ1 and SQ2 is helpful in getting a better understanding on the karst palae-ogeomorphologic features of the Maokou Formation and identifying the development of weathering curst karst reservoirs. Sequence stratigraphic correlation indicates that the Maokou Formation in the Sichuan Basin has "three intense and two weak" denuded regions, including two weak denuded regions in the SW Sichuan Basin and locally Eastern Sichuan Basin and three intense denuded regions in the Southern Sichuan Basin—Central Sichuan Basin, NE Sichuan Basin and NW Sichuan Basin (Fig. 5).
3.1. Stratigraphic development features of weak denuded regions
The Maokou Formation in the SW Sichuan Basin is slightest in erosion, especially in Ya'an—Yibin, SQ1 is well preserved and SQ2 is 120—150 m thick. Moreover, the weathering crust has been discovered under Emeishan basalt in the SW Sichuan Basin (Fig. 4c), indicating that the Maokou Formation had been eroded before the eruption of Emeishan basalt. Previous studies have also confirmed this point of view [9,10]. Compared with the other parts of the basin, the Mao-kou Formation in the SW Sichuan Basin is slightest in erosion, and most complete, within which, the TST of SQ2 (Mao 3 Member) has been preserved, but HST (the upper part of Mao
4 Member) has been eroded to varying degrees.
In Dazhu-Shizhu area in the Eastern Sichuan Basin, the Maokou Formation has also been slightly eroded, with SQ1 completely preserved and SQ2 being 110 m thick, within which, TST is left but HST is eroded locally.
3.2. Stratigraphic development features of intense denuded regions
From the SW Sichuan Basin to the Southern Sichuan Basin and Central Sichuan Basin, the erosion of the Maokou Formation gets worse. The southern center of this denuded region is Luzhou area. Previous studies show that [8,16,17] under the structural settings of the Dongwu Movement, the Luzhou area in the Southern Sichuan Basin was uplifted in local area, forming the prototype of Luzhou palaeohigh. The Luzhou palaeohigh was of NW trending, with its center in the north part of Luzhou City. Sequence stratigraphic correlation shows the SQ2 in the core of Luzhou palaeohigh (Mao 4 Member) is eroded out, and the closer to the center of palaeohigh, the more severe the erosion. For example, the top surface of the HST in SQ2, even Mao 2 Member is eroded. The Maokou Formation in the Central Sichuan Basin is also severely eroded, with SQ2 almost eroded out, for SQ1, the top of HST is partly eroded in the Moxi-Longniisi region, but the TST basal deposits of SQ2 is preserved in local area (such as Well Wangjia 1). The denuded region in the Central Sichuan Basin and Luzhou of Southern Sichuan Basin joined into a large intense denuded region.
Dazhou—Kaijing—Fengjie area in the NE Sichuan Basin and Jiange-Bazhong area in the NW Sichuan Basin are two intense denuded regions, where the Maokou Formation is severely eroded and the upper part of the HST of SQ1 is missing. However, in Tongjiang area between the two regions, the TST of SQ2 is partly preserved.
It should be noted that, in Chengdu—Mianyang area in the Western Sichuan Basin, the denudation of top Maokou Formation is not clear due to the large burial depth and the lack of drilling and geological data. The drilling data of Well Guanji shows that Mao 3 Member and part of Mao 4 Member are well preserved, namely SQ1 and the TST of SQ2 are completely preserved, but for HST, only its bottom is preserved. For the western Sichuan Basin, whether there is a third weak denudated region of the Maokou Formation and whether it is connected with the weak denudated region in the SW Sichuan Basin need to be further studied.
4. Control of sequence stratigraphic framework on source rocks and reservoirs
The study on sequence stratigraphy can clarify the distribution of source rocks and prospective reservoirs through an analysis of the eustatic sea level changes in vertical direction [18—20] and can provide guidance in oil and gas exploration. Studies show that, within the sequence stratigraphic framework of the Maokou Formation established by spectral trend attribute analysis technique, the development of source rocks and reservoirs follows an obvious pattern in the vertical direction, indicating that the development of source rocks and reservoirs was controlled by sequence stratigraphic framework.
4.1. Control on the development of source rocks
Petrological and geochemical analysis reveals that the completely preserved TST dark organic limestone of SQ1 can
Fig. 5. Sequence stratigraphic correlation and distribution of denudated regions of the Maokou Formation, Sichuan Basin (location of section line is shown in Fig. 1).
act as good source rocks. In petrology, compared with the limestone of HST in SQ1 in light color and coarse structure (medium-coarse grain), the limestone of TST is dark gray and gray black, fine in structure (micrite, fine-powder grain), interbed with marl and argillaceous lamina, zonal in distribution, and high in shale content, with fine wavy bedding. Its outcrop is characterized by "eyeball-eyelid" structure. The "eyelid" is equivalent to the matrix of "eyeball". Under microscope, it has orientated arrangement, with shale and fine bioclast intertwining together (Fig. 4d). Its genesis is related to deposition and diagenetic compaction [21]. In geochemistry, the SQ1 limestone has a TOC of 0—2.0%. In particular, TST has a higher TOC, generally between 0.5 and 2.0% and HST has a lower TOC, generally less than 0.5% (Fig. 6). The features of the kerogen types (amorphous), carbon isotope values d13C PDB distribution (-28%o to -33%o), chloroform bitumen "A" hydrocarbon composition (rich in hydrocarbons and dominated in saturated hydrocarbon) and infrared spectrum absorption peak of TST dark limestone indicate [22] the organic matter type is dominated by sapropelic-hybrid, which is mostly in high mature-over mature stage [1,22], with Ro of over 1.3%. Therefore, the organic matter of TST limestone in SQ1 can act as good gas source rocks for the reservoirs of Qixia Formation— Maokou Formation. Sequence correlation shows that this set of source rock is stable and pervasive in the entire basin.
4.2. Control on the development of reservoirs
Previous studies show that there are two kinds of effective reservoirs in the Maokou Formation, karst fracture-cavity
reservoirs and dolomite reservoirs [2,8], but their material base is the deposits of high energy grain bank. The vertical distribution of grain bank, strongly controlled by sequence boundaries, shows an apparent pattern in the sequence strat-igraphic framework, which means it predominately appears in HST, especially in the HST of SQ1 (Fig. 7), but rarely appears in TST due to the low water energy, insufficient wave filtering and high shale content of bioclastic limestone. The grain bank of TST is thin, with micrite matrix as main interstitial matter, whereas, thick and well-developed, lateral distribution of the grain bank of HST is related to sedimentary facies belt, but also controlled by the denudation degree of the Maokou Formation. Since SQ2 is eroded to varying degree in the entire basin, its HST is only left in local weak denudated regions and eroded out in intense denudated regions, even the
Fig. 6. Histogram of the distribution of the TOC of SQ1, Maokou Formation.
Fig. 7. Grain bank distribution within the sequence stratigraphic framework of the Maokou Formation, Sichuan Basin.
HST of SQ1 is severely eroded (such as Well Macao 2, Fig. 7). Thus, grain bank is rare or even disappears in the intense denudated regions of Northern Sichuan. The grain bank is dominated by tight bioclastic limestone with limited reservoir capacity (Fig. 4e) and can hardly act as effective oil and gas reservoirs. Its shale content is very low or even zero after being elutriated by high energy water and its cements are mainly composed of sparry cements after intense compaction and cementation. However, since the basin was uplifted and eroded in late Maokou Period, the grain bank was developed with dissolved pores and cavities as a result of intense karstification (Fig. 4f), and becomes good reservoir under the connection of the structural fractures of late stage. In addition, Emeishan basalt erupted and was accompanied by the Dongwu Movement when hydrothermal fluid upwelled along basement faults resulted in the dolomitization of HST grain bank limestone of SQ1, thereby, limestone with better reservoir property was formed, which is another important type of reservoir in the Maokou Formation. Well logging interpretation results of 41 wells drilled in Gao-mo area of Central Sichuan reveal that if the porosity of 2% is taken as the lower limit of effective reservoirs, the effective reservoir of the Maokou Formation is between 2.95 and 68.42 m thick, 18.6 m on average. For Well Gaoshi 1, 38.59 m gas layer, 29.82 m poor gas layer and 68.42 m effective reservoirs were interpreted, consistent with thick grain bank deposits developed in this well.
The analysis above indicates that the distribution of grain bank has some regularity within sequence stratigraphic framework, namely, it predominately occurs in HST. In addition, the distribution of grain bank has been further clarified based on the distribution of weak and intense denudated regions. The bioclastic limestone of shoal facies in the Maokou Formation is the material basis for the formation of effective
reservoirs. Therefore, effective reservoirs predominantly occur in the HST of SQ1.
5. Conclusions
1) With clear fundamental principle and geological significance, the spectral trend attribute analysis technique (INPEFA technique) combined with drilling and outcrop data can better identify the sequences of carbonate platforms. The sequence division of the Maokou Formation in the Sichuan Basin has proved that this technique has good application effect.
2) The Maokou Formation in the Sichuan Basin is divided into two third-order sequences. SQ1 includes Mao 1 Member-Mao 3 Member, within which, TST consists of Mao 1 Member and c Submember of Mao 2 Member, and HST consists of a and b Submembers of Mao 2 Member and Mao 3 Member; SQ2 is composed of Mao 4 Member, within which, TST and HST are located in its lower and upper part respectively.
3) Affected by the Dongwu Movement, the Maokou Formation in the basin has experienced erosion to varying degrees, forming three "intense and two weak" denuded regions, i.e. Southern Sichuan Basin—Central Sichuan Basin, NE Sichuan Basin and NW Sichuan Basin, and SW Sichuan Basin and locally Eastern Sichuan Basin. The upper part of SQ2 is slightly denuded in the two weak denuded regions, while almost denuded out in the three intense denuded regions.
4) The development of source rocks and reservoirs is strongly affected by sequence boundaries and system tracts. Source rocks predominately distribute in the TST of SQ1, while grain bank predominately occurs in HST, which forms effective reservoir after karstification and
dolomitization. As the HST of SQ2 is generally eroded, prospective reservoirs are predominately in the HST of SQ1 in the Sichuan Basin.
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