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Natural Gas Industry B 1 (2014) 72e77 www.elsevier.com/locate/ngib Research article Distribution pattern and exploration prospect of Longwangmiao Fm reservoirs in the -Longnu¨si Paleouplift, Basin

Li Yalin a,*, Wu Furong a, Liu Dingjin a, Peng Yong b, Chen Sheng a, Deng Xiaojiang a, Li Xiaojuan a, Gong Fuhua a, Chen Hua a, Gan Xiaming a

a Geophysical Prospecting Company of CNPC Chuanqing Drilling Engineering Co., Ltd., , Sichuan 610213, b Exploration Department of PetroChina Southwest Oil & Gas Field Company, Chengdu, Sichuan 610051, China

Received 7 January 2014; accepted 25 March 2014 Available online 1 November 2014

Abstract

The Longwangmiao Fm gas pool was found in 2012 in Moxi area in Leshan-Longnu¨si Paleouplift, central . Previous geological studies showed that the Lower Cambrian Longwangmiao Fm reservoir is mainly pore type reservoir of grain shoal facies, locally superimposed by the vuggy reservoir formed by Caledonian karstification, and the distribution of the reservoir has a critical control effect on the gas pool there. Therefore, the reservoir prediction in this area follows the approach of “looking for the overlap of shoal facies, karst, bright spots, and traps”. First, the favorable facies boundary and karstification effect range are defined based on sedimentary facies; on the basis of precise correlation of the top and bottom horizons of the reservoir, the favorable development area of the reservoir was then predicted according to the seismic response characteristics of “bright spots” of the Longwangmiao Fm reservoir; finally, favorable exploration areas were selected according to the effective configuration of reservoir and trap. Due to the horizontal change in reservoir and lithology, Longwangmiao Fm top has no uniform interface features on the seismic profile, making it difficult to trace. In the actual interpretation process, the underlying reference horizon is sought out first, then the bottom boundary of Longwangmiao Fm is correlated, and finally its top boundary is correlated. The prediction results indicated that the Longwangmiao reservoir distributes in band shape in a wide range around the Paleouplift; and the exploration prospects mainly involve three domains: lithologic-stratigraphic traps near the pinchout line of Longwangmiao Fm; lithologic traps formed by isolated shoal body; and structural and lithologic traps at the north flank of the Paleouplift. © 2014 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/3.0/).

Keywords: Central Sichuan Basin; Paleouplift; Early Cambrian; Reservoir; Distribution; Seismic exploration; Exploration zone

1. Introduction The present research results [8e10] show that the distri- bution of Longwangmiao Fm reservoirs has a critical control After over 50 years of petroleum exploration, a number of over the Longwangmiao Fm oil and gas pools. Therefore, the gas fields including Weiyuan and have been discovered key to the gas exploration in the area for the moment is to find in Sinian-Lower Palaeozoic strata of the Sichuan Basin [1e7]. out the distribution pattern of the Longwangmiao Fm reser- In 2012, high rate gas flow was tapped from Cambrian voirs, establish a set of exploration technologies in order to Longwangmiao Fm in several wells of the Central Sichuan hunt for new potential blocks. Palaeohigh area, making Longwangmiao Fm an important target of petroleum exploration in the Sichuan Basin. 2. Geological features of Longwangmiao Fm reservoirs in Central Sichuan Palaeohigh * Corresponding author. E-mail address: [email protected] (Li YL). The Leshan-Longnu¨si Palaeohigh was formed in Silurian- Peer review under responsibility of Sichuan Petroleum Administration. Pre- Permian, with the core in the southwestern Sichuan http://dx.doi.org/10.1016/j.ngib.2014.10.009 2352-8540/© 2014 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/3.0/). Li YL et al. / Natural Gas Industry B 1 (2014) 72e77 73

Basin, and axial in near EW trend, where the strata were denuded to Sinian Dengying Fm at most [11,12]. The zone from Weiyuan and Ziyang areas located at the core of the Palaeohigh to the pitching end is usually called the Central Sichuan Palaeohigh. Longwangmiao Fm, deposited at late stage of Early Cambrian, is composed of dolomite and argillaceous dolomite intercalated by minor sand shale lithologically; it is wide- spread in other areas except at the core area of the Leshan- Longnu¨si Palaeohigh where it is almost denuded away; except the area near the pinchout line in the Central Sichuan Palae- ohigh, it is generally 80e120 m thick, thinning from east to west, and in conformable contact with both the overlying Gaotai Fm and the underlying Canglangpu Fm. The overlying Gaotai Fm is primarily mudstone and argillaceous siltstone intercalated with limestone, and the underlying Canglangpu Fm sandstone, siltstone and mudstone intercalated with limy dolomite. Many research literatures [13e15] have discussion about shoal body, but there is hardly any report on the particle shoal body of Longwangmiao Fm in the Sichuan Basin. Previous studies show [16,17] that the Longwangmiao Fm reservoir rock is dominantly aplite-mesocrystalline dolomite with remnant particle (arene, oolite) structure, and the reservoir type is Fig. 1. Area affected by karstification in Longwangmiao Fm of Central Sichuan Palaeohigh. mainly fracture-pore (vug) type. The reservoir space consists of mainly intergranular pores and intercrystal pores in intraplat- form shoal facies, followed by dissolved pores and vugs in strong wave peak reflection at the top, and strong wave formed by karstification. In addition, well-developed fractures trough reflection at the bottom (Fig. 2). and dissolved fractures can not only serve as reservoir space, Longwangmiao Fm reservoirs are mostly developed in the but also largely improve reservoir permeability. Analysis middle and upper parts. If the reservoirs are developed, the shows that the Longwangmiao Fm reservoirs are strongly velocity will reduce substantially to 5500e5800 m/s, some- controlled by sedimentary facies, and they are mostly devel- times even as low as 5200 m/s, resulting in a big wave oped in the high energy shoal facies, whereas the dissolved impedance difference with the matrix. Therefore, reservoir pores (vugs) and fractures formed by karstification are of great intervals will form “bright” reflection in the interior of the significance to the improvement of reservoir quality. Longwangmiao Fm. Besides, because the development of The Longwangmiao Fm is dominantly evaporative- Longwangmiao Fm reservoir reduces the wave impedance restricted platform facies; intraplatform shoal is widely difference between the Longwangmiao Fm and the mudstone developed in Central Sichuan Basin, lagoon and dolomite at the bottom of Gaotai Fm, especially, when the reservoir is plateau facies are developed in other areas, indicating that the developed at the top of Longwangmiao Fm, the peak reflection favorable exploration scope of shoal facies reservoir is huge. at the top will be weakened, or even turns into wave trough Taking Well Moxi 17 where dissolved vugs are developed as (Fig. 3), making it difficult to correlate and interpret Long- the boundary, the area affected by Caledonian karstification in wangmiao Fm horizon, especially the top of Longwangmiao Central Sichuan Palaeohigh reaches 12600 km2, indicating Fm in the area without wells. that karst reservoirs cover a wide area (Fig. 1). Analysis of regional geologic and drilling data shows there was a maximum marine flooding surface in the depositional 3. Seismic identification of Longwangmiao Fm reservoirs period of Canglangpu Fm with a large set of mudstone and silty mudstone deposition, which, shown as a continuous It is the basic prerequisite of seismic identification of the strong event on the seismic profile, can be used as a regional reservoir to accurately calibrate the seismogeologic horizons marker bed for geological correlation (Fig. 4). As mentioned and conduct fine horizon correlation and interpretation. above, the lithology of Longwangmiao Fm is dominantly Mainly consisting of dolomites, the Longwangmiao Fm has dolomite, and there is big wave impedance difference between a matrix velocity of 6750e6900 m/s in general, so there are big Longwangmiao Fm and the mudstone of the underlying differences between the Longwangmiao Fm and the mudstone Canglangpu Fm, resulting in strong wave trough reflection at at the bottom of the overlying Gaotai Fm (with velocity of the bottom boundary; based on synthetic seismogram cali- 5400e5600 m/s) as well between the Longwangmiao Fm and bration, the strong wave trough above the Canglangpu Fm the mudstone and argillaceous siltstone of the underlying marker bed is the bottom of Longwangmiao Fm. The thickness Canglangpu Fm (with velocities of 5300e5800 m/s), resulting of Longwangmiao Fm is generally 80e120 m in Central 74 Li YL et al. / Natural Gas Industry B 1 (2014) 72e77

Fig. 2. Seismic response features of Longwangmiao Fm in Well Gaoshi 2. (1 ft ¼ 0.3048 m).

Sichuan Palaeohigh, so the t0 derived is 35e45 ms based on Moreover, the forward modeling results of geologic model inverse computation, therefore, the weak peak or trough designed based on the log data of Well Moxi-8 also prove that reflection 35e45 ms above the bottom of Longwangmiao Fm if the Longwangmiao Fm reservoir is not developed, its top is the top of Longwangmiao Fm. will be a strong wave peak, and its bottom will be a wave

Fig. 3. Seismic response features of Longwangmiao Fm in Well Moxi 12. (1 ft ¼ 0.3048 m). Li YL et al. / Natural Gas Industry B 1 (2014) 72e77 75

4. Distribution rule of the Longwangmiao Fm reservoirs

If the Longwangmiao Fm reservoirs were underdeveloped, the strong reflection at its top would affect the accuracy of reservoir prediction, therefore, when the maximum peak amplitude attribute is extracted, the top of the time window needs to be shifted downward below the strong crest at its top. Statistics on the maximum peak amplitudes of Longwangmiao Fm in wells of Gaoshiti-Moxi area showed that the maximum peak amplitudes after normalization are all larger than 110 in the areas where reservoir is developed (Fig. 6). Reservoir prediction study of Longwangmiao Fm shows that the Longwangmiao reservoirs spread across wide areas in ring shape around Central Sichuan Palaeohigh, and are distributed as big sheet shape in Moxi area, as band shape in Dazu, Wusheng, and Xichong, but are relatively poor in Fig. 4. Seismic interpretation profile of Cambrian Longwangmiao Fm. Gaoshiti area and the further north (Fig. 7). trough; when the reservoirs are developed in the middle and 5. Comprehensive prediction methods for “bright spots” upper parts of the Longwangmiao Fm, the energy of the event of the Longwangmiao Fm reservoirs at the top Longwangmiao Fm will weaken, and “bright spot” reflection will occur in the interior of the Longwangmiao Fm The Longwangmiao Fm gas pools in Moxi area are domi- (Fig. 5). nantly structural ones, where the development of reservoirs After the top and bottom of Longwangmiao Fm are accu- are obviously controlled by sedimentary facies. The shoal rately calibrated and interpreted, based on the fact that Long- facies or karst reservoirs are superimposed with local struc- wangmiao Fm reservoirs shall bring about such features as tures within certain scope which can usually form lithologic- weak reflection at the top, “bright spot” in the interior and structural combination traps, meanwhile, isolated shoal strong wave trough at the bottom, the maximum peak amplitude bodies can become lithologic traps, and all of them are the seismic attribute in the interior of the Longwangmiao Fm is focuses of future exploration. Based on the successful explo- used to conduct reservoir prediction of the Longwangmiao Fm. ration experience of Longwangmiao Fm reservoirs, the

Fig. 5. Geologic model and forward model of Longwangmiao Fm in Well Moxi-8. 76 Li YL et al. / Natural Gas Industry B 1 (2014) 72e77

the prerequisite of reservoir prediction is to conduct accurate horizon interpretation by using the regional marker beds, drilling data and regional geologic features jointly.

5.3. Searching for “bright spots”

Affected by not only sedimentary facies but also dolomi- tization and karstification, the Longwangmiao Fm reservoirs are distributed as discontinuous sheet or band shape. There- fore, in search of high energy shoal facies, karst reservoir development zone should be overlapped and considered too. Owing to the change in formation thickness and sedimentary facies, the “bright spots” prediction technique has ambiguity, Fig. 6. Statistics of maximum peak amplitude in the interior of the Long- so careful analysis and discrimination should be made in real wangmiao Fm. practice. integrated prediction process is summed up as “looking for the 5.4. Study on reservoir and trap relationship shoal facies, superimposed karst, bright spots, and traps”. The reservoirs predicted on the “bright spots” response 5.1. Using sedimentary facies study to define marginal characteristics are overlapped with the structural traps, litho- facies belt logic traps and composite lithologic-structural traps to seek out the favorable exploration targets by combination with the Longwangmiao Fm reservoirs are mainly developed in the generation, migration and accumulation of hydrocarbons. high energy shoal area of the intraplatform beach, therefore, in search of exploration targets, the favorable facies belts should 6. Analysis on Longwangmiao Fm exploration prospect be delineated based on sedimentary facies and seismic facies. The sedimentary facies of Weiyuan-Gaoshiti-Moxi-Longnu¨si- With the advance made in natural gas exploration of the Dazu area of Central Sichuan Basin where oil and gas dis- Longwangmiao Fm in Moxi area, looking for new exploration coveries have been made is of intraplatform beaches with huge targets ranks top in the next steps. While steadily carrying on exploration scope. the exploration of structural gas reservoirs of the Long- wangmiao Fm in Moxi and its peripheral areas, the future 5.2. Accurate interpretation of Longwangmiao Fm horizon exploration of the Longwangmiao Fm in Central Sichuan Basin will push forward into the following three domains. Because the development of Longwangmiao Fm reservoirs would make its horizon correlation and interpretation difficult, 6.1. Lithologic-stratigraphic hydrocarbon reservoirs near pinchout boundary

Because the strata near the Longwangmiao Fm pinchout boundary were directly exposed to surface or uplifted to the near surface in Caledonian, the strong atmospheric fresh water denudation causes strong karstification, and thus karst reser- voirs are likely to occur there; moreover, the direct caprock e Liangshan Fm mudstone on the top can act as effective seal- ing, making the formation of lithologic-stratigraphic traps possible.

6.2. Lithologic reservoirs formed by isolated shoal bodies

Longwangmiao Fm shoal bodies are formed in multiple phases, and the isolated lithologic reservoirs can be formed if the shoal bodies don't connected each other. If major discov- eries are made in this type of reservoirs, the exploration scope of Longwangmiao Fm will be largely extended, and the Shehong-Nanchong and Guang'an areas as well as the Hechuan Fig. 7. Development and distribution of the Longwangmiao Fm reservoirs in and Hebaochang areas in the south part of the Palaeohigh will Central Sichuan Palaeohigh. all be included in favorable exploration areas. Li YL et al. / Natural Gas Industry B 1 (2014) 72e77 77

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