Effects of igneous intrusions on source rock in the early diagenetic stage: A case study on Beipiao Formation in Jinyang Basin, Northeast China

2.1 Geological settings of Jinyang Basin and Beipiao Formation

The Jinyang basin is one of the four NE elongated early Mesozoic terrestrial basins in western Liaoning province, China. The Jinyang basin is bounded by the Nantianmen thrust fault to the west and border on the Beipiao basin and Chaoyang-Jianchang basin from north to south [36]. The Jinyang basin is about 200 km long and 40 km wide and covers an area about 7,400 km². The basin-fills mainly include a suite of Mesozoic terrestrial clastic deposits, volcanics, and pyroclastic rocks. It is the basin with the most developed Jurassic deposits in north China. Over 90% of the Jinyang basin is covered with middle and upper Jurassic volcano sedimentary successions, forming the most of the sedimentary sequences in this basin [37]. The Mesozoic basin-fill sequences in Jinyang basin start with lava and pyroclastic rocks and end with thick coarse clastic rocks and/or conglomerates, showing cyclic basin development controlled by geotectonic mechanism [38].

The Jinyang basin was formed as a result of the large-scale lithospheric thinning under extensional tectonic settings during early Mesozoic and then undergone compressional settings in late Jurassic time [39,40,41,42]. Located on the eastern North China Craton, it remained relatively quiescent from the late Paleoproterozoic until Mesozoic times. Voluminous Jurassic–Cretaceous volcanic rocks erupted and hence formed a series of small Mesozoic basins including the Jinyang basin (Figure 1). Early to late Jurassic mafic to intermediate volcanism was widespread across the Yanshan area, but reached its maximum intensity in the Late Jurassic and Early Cretaceous [43].

The early Jurassic Beipiao Formation, the focus of this study, was considered to be the most significant organic rich source rock in the Jinyang basin. Although there was extensive volcanism during Jurassic times, the Beipiao Formation was deposited during periods without much volcanism. The Beipiao Formation is a coal-bearing unit and consists of alternating fluvial lacustrine shale, sandy shale with sandstone, conglomerate, and coal-bearing layers. However, only few outcrops of Beipiao Formation can be found in the Jinyang basin, and most of the outcrops occur on the western area near the Nantianmen thrust fault, including Kuntouyingzi area, Wolong area, and the Sanbao coal pit. The oil and gas exploration in the Jinyang basin conformed that the Beipiao Formation has the potential to generate both oil and gas [35,44,45,46,47]. With three volcanic eruption-sedimentary cycles (Figure 1), the magmatic activity in the Jinyang Basin was relatively strong, and the effect of intrusive bodies on the thermal evolution of Beipiao source rock is significant. Igneous intrusions are common in Beipiao Formation, for example, the Wolong section and the Kuntouyingzi section.

The Wolong geological cross section is located in a synclinorium, which was formed during the early Indosinian-Yanshan orogeny stage as indicated in previous studies [37]. The synclinorium has an EW-trending axial trace with a length of 10 km (Figure 2). The west end and the north limbs were cut by faults, and the east side was unconformably overlained by early Crataceous Yixian Formation. The core of the synclinorium is composed of Beipiao Formation with several light gray to brown diorite porphyrite intrusive dykes. The limbs of the synclinorium are composed of the early Jurassic Xinglonggou Formation. The Dongkuntouyingzi geological section is located in Xiaogangou village, about 13 km east of Chaoyang City, with a length of 1.23 km. The main lithology of the Beipiao Formation in the section is green-gray thin-layered fine-grained sandstone interlayered by siltstone, yellow silty mudstone, and dark gray black mudstone. The sections provide a natural laboratory to study the interactive mechanism of the igneous intrusion body and the Beipiao source rock, which can provide insight into the thermal evolution history of the Beipiao source rock.

Figure 2

Location of the Wolong section and its stratigraphic column.

2.2 Relationship between host rock and igneous intrusions

Based on the geological cross section measurement, the strata along the Wolong cross section can be subdivided into 12 layers (Figure 2). The detail field observation of the lithological associations and sedimentary structures indicate sedimentary environments of alluvial fan, fan delta, and lacustrine deposits. There are four main black shale layers with a total thickness of about 98 m. The cumulative thickness of the Beipiao group calculated from the Wolong section is about 200 m. Black shale layers in this cross section mostly vary from 10 to 30 m (Figure 3a). The light gray-brown diorite porphyrite is located at the bottom of the ninth layer (Figure 3b) with an exposed thickness about 204 m, which exhibits parallel relationship with the bedding plane. The intrusive body shows a yellow-grey color in the outcrop and has typical spherical weathering characteristics (Figure 3c).

Figure 3

Representative field structures and thin section photomicrograph of diorite porphyry intrusion in the Wolong section. (a) black shales in the outcrop; (b) the contact zone of black shale and diorite porphyry intrusion; (c) the diorite porphyry intrusion show the spheroidal weathering appearance in the outcrop; (d) thin section photomicrograph of diorite porphyry intrusion.

4.1 Petrology of igneous intrusions

The intrusive dyke show the spheroidal weathering appearance with yellow to green yellow colors in the field. The petrological results indicate that the dyke is composed of altered quartz diorite porphyrite (Figure 3d). The phanerocrysts are mainly plagioclase and altered hornblende, which vary from 0.4 to 2.5 mm. The plagioclase is mostly subhedral to euhedral grains with platy or columnar form, and the surface is commonly altered with kaolinization, sericitization, and carbonatization. The hornblende grains are mostly metasomatized by carbonate minerals and biotite aggregates, retaining hornblende crystal form, present as pseudomorphs. The matrix is mainly composed of plagioclase, altered hornblende, biotite, and quartz. The mineral grains are less than 0.25 mm. The space between the disordered plagioclase grains are filled with other altered minerals. All these features suggest a hypabyssal intrusive face, which is also in accordance with the deduction that the igneous intrusions were emplaced during the early stage of diagenesis of the Beipiao Formation.

4.2 Changes in shale micro-texture

Based on the degree of metamorphism, we divide the samples into three groups, unaltered, mildly altered, and extremely altered. The unaltered samples are characterized by planar and wrinkled laminae in their microstructures (Figure 4a and b). The organic matters in the unaltered shales exhibit thin, continuous to discontinuous streaks. The mildly altered shale samples have darker organic matter compared with samples from the unaltered area (Figure 4c and d). Most of the organic matters in this area keep their original micro fabric structures. The samples of the extremely altered area nearly lost all the micro fabric details. The organic matters in this area have much small size and occur sporadically in the matrix of the silts (Figure 4e and f).

Figure 4

(a–f) Micrographs of samples in different zones, extremely altered samples (a and b), mildly altered samples (c and d), and unaltered samples (e and f).

4.3 Organic geochemistry of Beipiao shale

Various organic geochemical parameters tests were carried out for 154 drilling core samples, 164 shale samples from three measured geological sections, and 37 samples from four geological sections collected for comparisons and data analysis [48]. The organic rich source rocks from the Beipiao Formation, which were not affected by intrusive bodies in the Jinyang Basin were also analyzed (Table 1). The results show that the TOC value varies from 0.03% to 14.1% with an average of 1.19%. The pyrolysis hydrocarbon generational potential (S₁ + S₂) distributes in a range of 0.01–14.82 mg/g with an average of 1.61 mg/g. The T_max value ranges from 322°C to 563°C, with an average of 469.74°C. The R₀ value ranges from 0.6% to 1.54%, with an average of 0.94%. These parameters suggest that the Beipiao source rocks are in the mature stage and are basically fair to good source rocks, except for few samples, which are poor source rocks. However, it should be noted that high maturity and weathering may be cause of the extremely low potential for some samples, especially for samples in the exact vicinity of the intrusive body (Table 2). It can be concluded that the primary hydrocarbon potential is better than the analyzed results.

5.1 Width of altered zone

It is of great significance to understand the extent of thermal influence of intrusions on surrounding rocks. Numerous studies had illustrated that the thickness of contact metamorphism aureoles depends on factors such as thickness, size, temperature of magma, thermal properties of the host rock, thermal effects of water or metamorphism in the host rock, as well as the style of heat transfer [1,3,4,24,31,49,50]. The heat source from igneous intrusions had strong influence on the transformation of organic matter in the organic rich host rock and can also promote hydrocarbon generation process. In some cases, such as igneous intrusions emplaced into immature source rock, the igneous activity can generate the heat required to mature otherwise immature. Based on the different conditions and assumptions, previous studies indicate that the extent of thermal aureole is varied. In most cases studied, the thermal effect of igneous bodies intruded into sedimentary rocks is expected to be one to two times the thickness of the intrusion body [31,51,52,53,54]. Based on the published literatures and numerical modeling results, Aarnes et al. summarized that aureole thicknesses can vary from 30% to 250% of the sill thickness depending on host-rock temperature, sill thickness, and intrusion temperature [1]. Recently, Bulguroglu and Milkov reported that the relatively thin (<5 m) sills have aureole thicknesses of 276 to >1,000% of the sill thickness, significantly larger than the previously reported from most other locations affected by igneous intrusions [55]. However, these results were later questioned by other numerical modeling results, suggested that the aureole was exaggerated by low-resolution mesh, and concluded that an aureole thickness of 30%–200% of the sill thickness is applicable [54].

According to the variations of R₀ values of the samples analyzed, R₀ value of 0.95% is common across various sections and wells. Therefore, R₀ value of 0.95% is used as the background value to study the thermal aureole of the intrusive body for Wolong section. Similarly, the T_max value of 470°C is also used in this study to determine the extent of influence of the anomalous heat. The extremely altered zone is characterized by the complete loss of micro-texture because of charcoalization of the organic matter (Figure 4). Micro-textural details of the samples from the background area remain unaltered and keep almost all micro-textures, while samples from mildly altered zone record minor modifications of the texture. As indicated by the cross plot of T_max/R₀ and distance from the intrusive body (Figure 5), both the R₀ and T_max values reduce sharply in a few meters near to the intrusive body. As the distance from the contact zone reaches to about 200 m, both the R₀ and T_max values are stabilized and decrease to the regional background values (Figure 5). The cause of relatively high R₀ value in the range of 300–500 m is not well defined. Hence, the high R₀ values might include but not limited to weathering, heterogeneity of shales, and personal factors when measuring the vitrinite reflectance.

Figure 5

T_max values (a) and R_o values (b) as a function of distance from the dyke margin.

This width of the altered zone is consistent with the facts reported by other literature. As mentioned earlier, the extent of influence of the intrusive body is closely related to the scale of the intrusive body. In this study, the relatively thin aureole indicates that the heat brought by the magma is conducted mainly by a pure conductive model. Hydrothermal convection is limited in the host rock during the time of the igneous body intrusion and maybe related to the diagenetic stage of the surrounding source rocks. When the body in the Wolong section was formed (U–Pb age: 164.4 ± 1.5 Ma), the source rocks of the Beipiao Formation were still in the early diagenetic stage. The thermal conductivity is relatively low and specific heat capacity of the sedimentary deposits is high. The source rock in the vicinity of the intrusive body absorbed the majority of heat from the intrusive body, which led to the relatively narrow extent of influence of magma intrusion on the maturity of the source rock. In summary, the extent of influence of the anomalous heat of the intrusive rock on the surrounding rock is not only affected by the initial temperature and scale of the intrusive body but also related to the diagenetic stage of the surrounding rock.

5.2 Effects of igneous intrusions on hydrocarbon generation

With the increasing thermal maturity, the TOC values of source rocks will be progressively diminished due to generation and discharge of hydrocarbon from source rocks. When comparing samples from the two sections, it is observed that all samples have relatively high TOC value, the closer the samples to the intrusive body, the lower the hydrocarbon generation potential (S₁ + S₂) and hydrogen index (HI) (Figure 6). In addition, for samples in the exact vicinity of the intrusive body, the chloroform asphalt “A” content and total hydrocarbon content are lower than 0.015% and 100 µg/g, respectively, indicating nonhydrocarbon source rock. These results suggest that the anomalous heat from the intrusive body can not only promote the maturity process of source rocks but also can reduce the abundance of the residual organic matter in the source rocks and can even make nonhydrocarbon source rocks.

Figure 6

Cross plot of S₁ + S₂ (a) and HI (b) versus distance from the intrusion body.

Judging by the PI-T_max diagram (Figure 7), most of the samples have entered the oil window, and large-scale hydrocarbon expulsion had took place. There is not an obvious trend for the T_max value and PI value, where a clearly positive relationship is often seen in other areas. Our explanation for this phenomenon is migration of hydrocarbon in the shales. For samples nearer (<100 m) to the intrusive body, almost all the generation potentials had been exhausted and all have very low S₂ values. However, there may be still some amount of free hydrocarbon (S₁) present and hence may yield relatively high PI in the nearby area. For samples in the oil window, a clear negative relationship between the PI and distance to the contact zone is seen (Figure 7), i.e., samples in this area have abnormally high free hydrocarbon. It can be concluded that these hydrocarbons were migrated from the high maturity area near the contact zone due to the fluid overpressure [57]. The heating effect is negligible at distance faraway from the intrusive body, where due to relatively low maturity level, only limited free hydrocarbons were generated and hence have the lowest PI values. If this deduction is true, then it can be inferred that the igneous intrusion in this area has beneficial effect on the accumulation and storage properties of the shales. In fact, these phenomena are not rare and have been reported in many other areas. Saghafi et al. reported that in South African coal seams, the effect of the igneous intrusions had enhanced the gas release and storage of these coals [58]. In the Illinois Basin, in the altered zone, strong reduction in mesopore and micropore volumes was observed, which has a negative effect on the adsorption of gas [18]. However, in the Daxing Mine, China, increased porosity, pore volume, and average pore diameter were found in the thermally altered area due to igneous intrusions, and the transport capacity is increased [59]. Based on study of changes of coal organic composition and pores/fractures resulting from igneous intrusions, Yao and Liu pointed out that influence of igneous intrusion on coal pores and fractures vary significantly depending on the intrusion type, the rank after the intrusion, and the nature of the sediments being intruded [22]. Correspondingly, as a main topic of shale gas evaluation, it has been reported that due to the generation and expulsion of hydrocarbons from organic matters, the pore volumes generally show a positive relationship with thermal maturity [60,61,62,63,64,65]. Combined with the fact that hydrocarbon generation conversion rates are inversely proportional to the distance from the intrusion for samples in the oil-producing window, we can prudently draw the deduction that the additional free hydrocarbon has migrated from the high maturity areas.

Figure 7

Plot of PI versus T_max values of Beipiao source rock from Wolong and Kuntouyingzi sections. Modified from Waples [56].

For the samples in the oil-producing window, their hydrocarbon generation conversion rates are inversely proportional to the distance from the intrusion to the samples (Figure 7). Three samples about 500–800 m from the intrusive body have a very low conversion stage. The results show that due to the influence of the anomalous heat from the intrusive body, the hydrocarbon conversion rate of the source rocks is greatly increased. In this case, the igneous intrusions in the Jinyang Basin might have promoted the generation and expulsion of hydrocarbons from the source rocks and greatly improved the production index. These results can provide useful evidences for the understanding of oil and gas formation processes in the Jinyang Basin. This observation can also provide useful basis for the prediction of resource potential and promising exploration blocks in the Jinyang basin.