Source and element background of desorbed gas from the siderite-bearing mudstone in northern Guizhou Province

Longtan shale is widely developed in Guizhou Province and is an important part of the unconventional natu- ral gas for the development in coal-bearing formations. Some mudstones have special characteristics due to some sider- ite minerals contained. This paper takes the Upper Permian Longtan shale in the northern Guizhou as an example to examine the coal-bearing mudstone reservoir. Based on the canister desorption,well logs and test results of siderite- bearing mudstones (core samples from LC-1 well),the authors present systematic pore structure,inorganic and Rock- Eval analyses of the mudstones to examine their gas content,reservoir properties,depositional environ-ment and origin. The results show that the Longtan shale contains type Ⅲ gas-prone organic matter with an over-mature stage and has total organic carbon values ranging between 0. 90% and 2. 71% . Carbon isotope data clearly indicate the terrestrial or- ganic matter source dominating the Upper Longtan formation. Desorbed gas content ( obtained by canister desorption, dominated by methane) of core samples ranges from 0. 08 to 7. 79 m3 / t with an average of 1. 60 m3 / t. The mudstones are mainly composed of clays (35% ) and quartz (15. 7% ),and most of them contained high siderite and dolomite with an average of 38. 1% and 9. 5% ,respectively. Desorbed gas content of core samples are positively correlated with TOC but negatively correlated with quartz and clay mineral contents,BET specific surface areas and BJH total pore volumes measured by gas injection method,which suggests that the organic matter is the main material source for de- sorbed gas,but the storage space is not provided by mesopores or micropores. However,the negative correlation be- tween siderite content and specific area with pore volume (obtained by gas injection method),suggesting that the in- tercrystalline pore of siderite is insufficient to provide the storage space for the occurrence of the desorbed gas. The dis- tribution of siderite around the organic matter is stratified,botryoidal,lenticular or tuberculous,and the stratigraphic continuity remains intact. The siderite surrounding the organic matter,as analogous to a “micro-trap” inside the mud- stone,blocks the hydrocarbon gas migration in the organic matter. This may be the main reason for the positive correla- tion between desorption and siderite content. The organic matter always adsorbed and stored the hydrocarbon gas genera- ted and sealed by “micro-trap”. With the increase of organic matter and “micro-trap”,the amount of enclosed gas in- creases. The siderite-bearing mudstone with filled gas may further reach as an overpressure reservoir for a certain pres- sure gas sealing. Furthermore,the siderite-bearing mudstone can have independent “micro-gas system” for the lacking of gas migration and exchange with adjacent coal seams. The reducing conditions during the deposition of Longtan formation for the response of elemental geochemical index is closely interrelated to the fluctuation of sea level,indicating that the typical sedimentary environment provides favorable conditions for the siderite. The desorbed gas,siderite and organic car- bon content change regularly with the process of sea level falling and rising from bottom to top of the Upper Permian,in- dicating that the siderite and organic matter deposited in tidal flat-lagoon with relatively stable Eh and pH values and limited water circulation. The high ferric ions brought into by sand interstitial water chemically react with reducing gas from the reduction of organic matter to create hydrocarbon gas. The gas sealed by “micro-trap” is stored as adsorbed gas on organic matter,suggesting a controlling effect on the desorbed gas of siderite-bearing mudstone.