Distributive characteristics of reservoirs and exploration potential associated with intrusive rocks of Yingcheng Formation in Yingtai rift depression, NE China

TANG Huafeng, KONG Tan, ZHAO Huiand GAO Youfeng

1.CollegeofEarthSciences,JilinUniversity,Changchun130026,China;2.ExplorationandDevelopmentResearchInstituteofJilinOilfield,Songyuan138000,Jilin,China;3.ResearchCenterofPalaeontology&StratigraphyofJilinUniversity,Changchun130061,China



Distributive characteristics of reservoirs and exploration potential associated with intrusive rocks of Yingcheng Formation in Yingtai rift depression, NE China

TANG Huafeng1, KONG Tan1, ZHAO Hui2and GAO Youfeng3

1.CollegeofEarthSciences,JilinUniversity,Changchun130026,China;2.ExplorationandDevelopmentResearchInstituteofJilinOilfield,Songyuan138000,Jilin,China;3.ResearchCenterofPalaeontology&StratigraphyofJilinUniversity,Changchun130061,China

Abstract:Petroleum geologists have paid great attentions to the volcanic reservoirs of Songliao Basin in NE China. There are plenty of subvolcanic rocks in the Songliao Basin accompanying the Early Cretaceous Yingcheng Formation. The logging data show the good reservoir potential of these intrusive rocks but the distribution characteristics and formation mechanisms of these reservoirs are not clearly understood. Based on the previous studies by using coring, cuts and logging data of Yingtai rift depression，the reservoirs’ characteristics of intrusive rocks are presented. There are two types of intrusive rocks namely the syenodiorite-porphyrite and diabase which occur as laccolith and/or sill, both having the characteristics of low gamma and high density with little primary porosity and permeability. The prevalent reservoir porosity is the secondary porosity, such as spongy/cavernous pore, tectonic fracture. The laboratory data of porosity of diabase can reach 6.7%, but the permeability is less than 0.6×10-3μm2, median pressure is high, indicating that the pore throat of this kind reservoir is small. The maximum logging porosity is about 12%. The change of porosity does not correlate to the buried depth. It is the major significant differences in the distributive characteristics compared to the normal sedimentary rock reservoirs. Most of intrusive rocks underwent alteration diagenesis whilst some were subjected to precipitation diagenesis. The spongy and cavernous pore can be formed during the alteration processes of plagioclase to illite and pyroxene to chlorite. The secondary porosity is greatly correlated with the alteration intensity of matrix, plagioclase and pyroxene. There are pyroxenes and more plagioclases in diabase， which cause the higher alteration intensity than the syenodiorite-porphyrites in the same acid fluid. So the porosity of diabase is higher than that of syenodiorite-porphyrites. The top or/and bottom part of intrusive rocks develop the higher porosity. Because those parts are easy to contact formation fluid, and the shrink fractures give the more surface for reaction between fluid and rock. The porosity of intrusive rocks is same to the volcanic rocks in Yingtai rift depression and Xujiaweizi rift depression which bear the prolific gas. It suggests good reservoir potential. Intrusive rocks are hosted by the dark mudstone which indicates semi-deep and deep lake facies belt.

Key words:Songliao Basin; Yingcheng Formation; intrusive rocks; porosity; alteration diagenesis

1Introduction

Since 2002, the proven gas reserves of Early Cretaceous Yingcheng Formation in Songliao Basin has exceed 600×109m3(Feng, 2008; Liuetal., 2010), occurring mainly in volcanic reservoirs with subordinate amounts of gases in conglomeratic reservoirs (Ca.10%). The gas production from igneous rock reservoirs have been reported in many basins, such as in the Pacific Rim structural belt, e.g. California (Nakata, 1980), Argentina (Sruoga and Rubinstein, 2004; Rodriguezetal., 2009), far East Russian (Levin, 1995), Japan (Sakataetal., 1989; Mitsuhataetal., 1999), China(Lvetal., 2004; Wangetal., 2007a; Feng, 2008; Liuetal.,2010; Xieetal., 2010) and Southeastern Asia (Sembodo, 1973). The Volcanic rocks develop primary and secondary porosity. These are intershard and intrapumice, intracrystalline sieve or moldic, vesicular and gas pipes, interflow laminar/tension cracks cavitation, interclast, shattered crystalline, spongy to cavernous, lacy, drusy, secondary sieve to moldic, tectonic fracture, quench fracture and drusy breccias(Luoetal., 1996; Liuetal., 2003; Zhaoetal., 2004; Sruogaetal., 2007; Liuetal., 2013). The distribution of volcanic primary porosity is controlled by volcanic edifices, lithofacies and the stratigraphic boundary (Wangetal., 2003; Zouetal., 2011; Tangetal., 2008, 2010, 2011, 2013). The formation of secondary porosity is controlled by diagenetic processes (Gaoetal., 2007, 2013; Liuetal., 2010; Huangetal., 2010; Zhengetal., 2010). The intrusive rocks develop primary fracture, induced fracture and solution cave (Guetal., 2002; Wuetal., 2006) which is valuable to exploration of oil and gas in the intrusive rocks in Liaohe Basin and Northern East China Sea Shelf Basin (Wuetal., 2003; Cukuretal., 2010; Guoetal., 2013). However, the characteristics and formation mechanisms of hydrocarbon reservoir of intrusive rocks in Songliao Basin is not clearly understood.

The main objectives of this manuscript are to document the features and reservoir-forming mechanism of intrusive rocks in the Songliao Basin in NE China. Detailed analysis of porosity of these intrusive rocks provides significant indications for its exploration potential in the Songliao Basin and the adjacent area.

2Geological setting

The tectonic evolution of the Mesozoic-Cenozoic Songliao Basin of NE China can be roughly divided into three stages namely rift, sag and shrinkage stages(Wangetal., 2007b). Approximately 45 rift depressions fall under the rift stage. The Yingtai rift depression extends more than 1,800 km2(695 mi2) in the middle-west of Songliao Basin (Fig.1). Overlying the Paleozoic metamorphic basement is the rift depression infilling of Cretaceous rift-related volcanic sequence. The Yingcheng Formation is also widespread in the other rift depressions and it forms a large igneous province (Jiaetal., 2007; Caietal., 2012), extending over an estimated area of 0.85×106km2(3.28×105mi2). The volcanic activity, which extended over a time span of 25Ma. (135-110 Ma) in the Cretaceous (Wangetal., 1995, 2000), showed an eastward migration, reflecting tectonic changes during the different stages of collision between Eurasian plate and Siberia plate. The emplacement of this huge volcanic unit is coeval with a regional lithospherical extension regime active since the Jurassic (Yangetal., 2005). The rift system is composed of inverse oriented half grabens, controlled by northeast-southwest trending main faults (Liuetal., 1992; Geetal., 2012). The Yingtai rift depression is dominated by voluminous rhyolite, with associated diabase, diorite porphyrite, minor intermediate lavas, and tuffs. The Yingtai rift depression is terminated in the west and overlapped in the east. Wukeshu fault with NW and nearly N-S trending is in the west boundary. It reversed after the Yingcheng period, leading to uplifting and denudation of Yingcheng Formation (Fig.1). The elevation in Fig.1b is the depth of top boundary of Yingcheng Formation.

Fig.1　Pilot map and characteristics of Yingtai rift depression

3Methodology

The distribution characteristics of intrusive rocks were analysed by integration analysis of petrology, logging and 3D seismic. The void spaces were assessed through a combination of megascopic observations, thin section, thin section of resin-impregnated and Scanning Electron Microscope (SEM) studies of samples from 11m core and 12 pieces sidewall cores and selected 71 cuts of intrusive rocks from Yingtai rift depression. Petrophysical analyses were undertaken on core samples to measure the porosity and permeability. Porosity was measured in a constant-volume cell employing a mercury (Hg) porosimeter. The porosity of logging is evaluated by logging company of Jilin oil field. The density logging is calibrated by using the mudstone of Yingcheng formation. All the segments of diameter boring for evaluate porosity and calibrate density logging is normal size. The vertical distribution characteristics of porosity were analyzed by the laboratory data and logging data. The porosity enhancing was estimated by image analyzing with SEM photographs.

4Features of the intrusive rocks

Of the 21 wells in the study area, 9 wells reveal 2 kinds of intrusive rocks. The syenodiorite-porphyrite and diabase which mainly are distributed in wells block L1 in southern part of the Yingtai rift depression (Fig.1). The detailed descriptions are as follows.

4.1Syenodiorite-porphyrite

The wells L1-1, L1-4, L101 and L7 reveal that the syenodiorite-porphyrite has 10 layers and 308 meters thick, and the thickness of a single layer is from 4 to 102 meters. Syenodiorite-porphyrite is multi-spot and glomerophyric in texture. Alkalifeldspar and plagioclase phenocrysts predominate with small amounts of hornblende. The groundmass consists of small amounts of microcrystallization and cryptocrystalline plagioclase. Illitization of syenodiorite-porphyrite is obvious (Fig.2A/I). Syenodiorite-porphyrite has the features of low gamma and high density, the gamma

Fig.2　Geological and geophysical characteristics of intrusive rocks of Yingcheng Formation in Yingtai rift depression

curve shows blocky pattern and the density curve shows blocky or jagged pattern (Fig.2A/II). The seismic facies across the well L1-1 is mound-shape, worm-waveform likeness, good continuance, medium-high amplitude, low frequency. It is similar to the laccolite (Fig.2A/III, IV). The seismic facies across the well L101 is slabby-shape, medium continuance, high amplitude, low frequency, with characteristics similar to that of a sill. The thin bedded syenodiorite-porphyrite of the well L1-4 has no responses in 3D seismic refection section; probably due to its small scale and occurrence as dike or apophyse.

4.2Diabase

The well L105, L7 and L1-1 reveals that the diabase has 7 layers and 527 meters thick, and the single layer thickness is from 4 to 177 meters. The diabase is composed of plagioclase and augite, with a diabasic texture. It is clear that the augite altered into chlorite and other clay minerals, and calcite filling some parts (Fig.2B/I). Diabase also has the features of low gamma and high density. Both the gamma and density curve shows blocky or jagged pattern (Fig.2B/II). The wall caving is severe from 3050 m to 3140 m in well L1. There are two segments of syenodiorite-porphyrite in well L1; the seismic facies of upper segment across the well L1 is mound-tabular shape, section below shows the features of mound shape, good continuance, worm-waveform likeness, medium-good continuance, medium-low amplitude, and low frequency. It is similar to both the laccolite and sill, and in high-medium amplitude, low frequency, so it is similar to laccolite. The seismic facies across the well L105 is mound-tabular shape, worm-waveform likeness, medium-good continuance, medium-low amplitude, low-medium frequency. So it is similar to both the laccolite and sill (Fig.2B/III, IV).

5Characteristics of porosity

5.1Characteristics of void space

Both cores and cuts reveal intrusive rocks in Yingtai depression develop the primary void space and secondary void space. The primary pores of intrusive rocks are not very much pronounced compared to the secondary pores.

The primary pore is only found shrink fracture. Fig.4-P1 show the jagged shrink fractures develop in the top part of intrusive. The width is about 1mm, length is 1m and distance between the adjacent two fractures is several centimeters. The filling intensity is 90%, the clay minerals and bitum been the filling matter.

The secondary pore is abundant, such as spongy pore, cavernous pore, secondary sieve pore and tectonic fracture. The cavernous of diabase has the characteristics of irregular shape, uneven size and no filling. The red part in Fig.3b is the secondary sieve pore in diabase(the red part) that distribute in all the sidewall core. The Fig.4-P1 reveals the hydrothermal-magma fracture. The width is Ca. 0.5 cm, lengthe is more than 10 cm. The filling intensity is 95%, the hydrothermal- magma solution been the filling matter. The Fig.3c and Fig.4-P3 reveal the tectonic fracture. The width is Ca. 1--2 cm, length is more than 10cm. The filling intensity is same to shrink fracture, the calcite been the filling mineral. So, the primary and secondary fracture supports little porosity. The secondary spongy/cavernous pore is the mainly contributor of porosity.

Fig.3　Characteristics of pores and fracture of intrusive rocks of Yingcheng Formation in Yingtai rift depression

Fig.4　Distribution characteristics of intrusive rock reservoir of Yingtai rift depression

5.2Characteristics of porosity

The laboratory data of porosity of diabase can reach 6.7%, but the permeability is less than 0.6×10-3μm2, median pressure is high, indicating that the pore throat of this kind reservoir is small (Table 1). The well logging data shows that the maximum porosity is 12.1%.According to the 7 segments (No.①～⑦) of 4 wells, it reveals the distributing characteristics of the intrusive rock’s reservoir (Fig.4).

First and foremost, the change of porosity does not correlate to the buried depth. Most of well segment of intrusive rocks has the higher porosity at the top and/or bottom part. The following are the changes in porosity trends. The well segment ③, ⑤ and ⑦ become higher with the increasing burial depth; The well segment ③ gets lower; the well segment① become lower and then gradually increases; the porosity of well segment④ has no obvious change; the porosity of well segment⑦ first becomes higher and then gradually decrease. So porosity of intrusive rocks is greatly correlated with outer cooling surface.

In addition, the porosity is correlated to the lithology and well location too. Firstly, the logging porosity of diabase is higher than that of syenodiorite-porphyrite’s. The logging porosity of syenodiorite-porphyrite is from 0.1% to 5.1%, the average is 2.8 %. That of diabase is from 0.5% to 12.1%, average is 6.1%. So the porosity of diabase is higher than that of syenodiorite-porphyrite. Secondly, the logging porosity of same lithology is correlated to well location. For example, the porosity of diabase in well L105 is from 6% to 12.1%. But that of L7 is only from 0.5 to 4.7%. As another sample, the porosity of syenodiorite-porphyrite of well L1-1 is from 0.1 to 5.1%. But that of well L7 is only from 0.1 to 3.9%. So the porosity is correlated to the well location.

Table 1　Features of the subvolcanic reservoir of Yingcheng Formation in Yingtai rift depression

6Discussions

6.1 Reservoir formation

Several processes take place after the intrusive rocks were intruded to country rocks. They go through two stages: (1) the cooling history stage, which includes crack and joint; and (2) the post cooling history stage, which can embraces quench fragmentation, hydrothermal alteration, precipitation, weathering and tectonic deformation (McPhieetal.,1993; Guetal., 2002; Wuetal., 2006). The alteration processes weathering and tectonic deformation contribute to enhance the total porosity(McPhieetal.,1993; Poursoltani and Gibling, 2011), precipitation processes reduce porosity and permeability (Dobsonetal., 2003).

Yingtai rift depression cores and cuts provide strong evidence for shrink fracture (Fig.4-P1), alteration (Fig.3a, b; Fig.4), tectonic reformation and precipitation (Fig.3c, Fig.4-P3). The alteration of matrix, plagioclase and pyroxene generate the secondary void space. Associated porosity may be spongy when the pores are small and medium sizes, or cavernous when they are large. That is the most important porosity in intrusive rocks because of high filling intensity of shrink and tectonic fracture. The alkalifeldspar undergo litter or no alteration. Following is detail explanation of the alteration process of plagioclase and pyroxene.

Alteration of plagioclase: The core of the well L1-1 at 2046.8 m (measure depth) reveals the features of secondary pores generated by alteration of plagioclase. The spongy pores in syenodiorite-porphyrite are formed during the alteration of plagioclase to illite (Fig.5A). This process may involve two stages. Firstly, the plagioclase is altered to kaolinite in acid formation fluid that was generated by thermal evolution of coal and mudstone of Yingcheng Formation. Secondly, the kaolinite changed into illite when the formation fluid transferred from acid to alkaline because the acid was consumed and the organic did not generate the acid during postmature stage.

The characteristics of spongy pore of plagioclase alteration is honeycomb (Fig.5A). The long diameter of spongy pore is between 0.31 μm and 3.07 μm, and mostly is between 0.61 μm and 1.54 μm. And these pores are the greatest contributor to porosity (Fig.6A). The spongy pores in illite can increase porosity from 6.6 to 8.2% as evidenced in Table 2.

Alteration of pyroxene: Fig5B shows the features of spongy pores generated by alteration of pyroxene. The spongy pores are formed during the alteration of pyroxene to chlorite (Fig.5B). These processes probably involve two stages. The formation fluid is same to that of syenodiorite-porphyrite. It is different in the mineral transforming. Firstly, the pyroxene was altered to saponite in acid condition. Secondly, the saponite changed into chlorite in alkaline environment.

The characteristic of spongy pore of pyroxene alteration is schistose (Fig.5B). The long diameter of spongy pore is between 0.88 μm and 8.12 μm, and mostly is between 2.2 μm and 5.54 μm. And these pores are the greatest contributor to porosity (Fig.6B). The spongy pores in chlorite can increase porosity from 6.5 to 8.5% as evidenced in Table 2.

Fig.5　Characteristics of alteration diagenesis of intrusive rocks of Yingcheng Formation in Yingtai rift depression

Table 2　Reservoir effect of alternation in intrusive rocks of Yingtai rift depression

Notes: the incremental porosity is evaluated by PIA.

Fig.6　Characteristics of spongy pores and porosity increased of intrusive rocks alteration of Yingcheng Formation in Yingtai rift depression

6.2Distribution model of porosity in single subvolcanics body

Fig.4 shows three types of alteration intensity in intrusive rock. Firstly, the plagioclase in syenodiorite-porphyrite underwent the weak-middle alteration, and the matrix underwent the middle alteration. Such as the Fig.4-P2 shows. Secondly, the pyroxene in diabase underwent the middle alteration and plagioclase underwent the weak alteration. Such as Fig.4-P5, P6 shows Ca.30% and Ca.60% pyroxene was altered into chlorite, and the litter plagioclase was altered. Thirdly, the pyroxene in diabase underwent the strong alteration and plagioclase underwent the middle alteration. Such as Fig.4-P7, P8 shows almost pyroxene was altered into chlorite and some clay, and most of plagioclase was altered clay. It is obviously that well segments (No.③ and ⑤) with strong alteration of plagioclase, pyroxene and matrix correlate the higher porosity, and the well segments (No.①, ⑥ and ⑦) with weak alteration of matrix, plagioclase correlate the low porosity (Fig.4).

Also, the rule that the porosity is greatly correlated to the alter intensity is proved by the single intrusive rock layer. The top and/or bottom part is easy to contact formation fluid, and the shrink fractures of outer layer can give the more surface (Fig.4-P1) for reaction between fluid and rock. That helps to enhance the alteration intensity. The pictures of well segments ① and ② in Fig.4 show the top and bottom part of single layer intrusive rocks underwent the higher alteration than the middle part, and the top and bottom part is higher porosity. The Pictures of well segment ⑤ in Fig.4 present the lower part of single layer intrusive rocks underwent the higher alteration, and the upper part is high porosity. So the top and/or bottom part have the higher porosity.

In addition, the faults and fissures have the same characteristics. The maximum porosity is present at the intersection of the fault/fissure and the cooled surface. Therefore, where there are more faults or fissures, there are more secondary pore and higher porosity. Fig.7 shows the ideal model of porosity distribution. This model is partly same to the Liaohe basin’s with the distinguished difference in void space (Guetal., 2002; Wuetal., 2006).

At last, the pyroxene is easier to alter than the plagioclase; the plagioclase is easier to alter than alkalifeldspar in same acid environment. There is 5% pyroxene and 70% plagioclase in diabase. But there is no pyroxene and only 30% phenocryst is plagioclase in syenodiorite-porphyrites. The alteration intensity of diabase is higher than that of syenodiorite-porphyrites. So, the porosity of diabase is higher than that of syenodior-ite-porphyrites in Yingtai rift depression.

Fig.7　Sketch map of distribution model of porosity in single intrusive rock mass

6.3Exploration potential

The increased porosity by alteration diagenesis is similar to the massive lavas with sieved K-feldespar in South Patagonia, Argentina (Sruogaetal., 2004, 2007), and more than the porosity (measure value 4%) in highly altered tuffs as lacy (Stimacetal., 2004). That evidence can also partly be supported by the logging porosity of intrusive rocks reaching to 17%. The porosity of intrusive rocks is same to the volcanic rocks in Yingtai rift depression (Gaoetal., 2013) and Xujiaweizi rift depression which bear the commercial gas(Feng, 2008). In conclusion, intrusive rocks in Yingtai rift depression are valuable to exploration. Especially, it is hosted by the dark mudstone which is semi-deep and deep lake facies belt.

7Conclusions

There are two kinds of intrusive rocks of Yingtai rift depression in Songliao Basin. They are syenodiorite-porphyrite and diabase, which occur mainly as laccolite and sill, and small amounts of dikes or apophyses. The intrusive rocks have the characteristics of low gamma and high density, both gamma and density curve shows blocky or jagged pattern. Seismic facies is mound-plank shape, worm-waveform likeness, medium-good continuance, medium-weak amplitude, low-medium frequency.

The primary pores of intrusive rocks of Yingtai faulted-depression are not pronounced compare to the secondary alteration-dissolution pores and fractures, such as the spongy/cavernous pore. The pore throat of this reservoir is small. The maximum logging porosity is about 17%. The porosity of diabase is higher than that of syenodiorite-porphyrites. The porosity is great correlated with surface of intrusive rocks and well location.

The alteration is common in intrusive rocks. The probability of precipitation and replacement is lower, usually not symbiotic. The secondary porosity of intrusive rocks is greatly correlated with the alteration intensity of matrix, plagioclase and pyroxene. The top and/or bottom part of intrusive rocks is easy to contact formation fluid, and the shrink fractures give the more surface for reaction between fluid and rock. So the top and/or bottom part of intrusive rocks develop the higher porosity. The pyroxene is easier to have high alteration degree than the plagioclase in same fluid. So the porosity of diabase is higher than that of syenodiorite-porphyrites. The highly potential area for exploration is the intrusive rock in semi-deep and deep lake facies belt.

Acknowledgments

Thanks to Professor LIU Wanzhu for lithological descriptions and very thoughtful suggestions that helped improve this manuscript. Thanks to Phiri Cryton for assistance in grammatical style. Thanks also to WU Yanhui, HUANG Yulong, ZHANG Yan and BAI Bin for sample collection and laboratory testing. This study was supported by the National Natural Science Foundation of China (41002038), the National Major Fundamental Research and Development Projects (2012CB822002 and 2009CB219304).

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doi:10.3969/j.issn.1673-9736.2016.01.03

Article ID: 1673-9736(2016)01-0013-13

Received 7 April 2015, accepted 18 June 2015

Supported by Projects of the National Natural Science Foundation of China (41002038), and the National Major Fundamental Research and Development Projects (Nos. 2012CB822002, 2009CB219304).