dlis is a binary file format for well logs, developed by Schlumberger in the late 80’s and published by the American Petroleum Institute (API) in 1991. In 1998 the stewardship was handed off to Petrotechnical Open Software Corporation (POSC), now known as energistics 
When developing dlis standard, the main emphasis was put on easy writing. The files are structured in such a way that data can be written directly while acquiring the logs. This is very handy for the producers of the files, and equally tedious for the consumer that wants to read them later on. Additionally it is a very tolerant standard. It specifies general data-structures within the file, such as channels and frames, but allows the producers to modify these to fit their needs. It also allows for completely new structures, not defined by the standard itself, such as vendor-specific object-types. This all sums up to a fairly complex standard with a lot of quirks and weirdness to it. It is safe to say that dlis a particularly difficult format to work with.