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Despite the abiotic theory referred to above, it is virtually certain that all the petroleum and natural gas in the earth's crust is of fossil origin (in fact, it would be a thrilling discovery if any prebiotic hydrocarbon deposits were ever found -- it would be unsurpassed evidence of Earth's primordial condition).
Petroleum (like coal) consists of a mixture of very many different compounds of hydrogen and carbon, with some sulfur, nitrogen, and oxygen compounds thrown in. Some of these compounds have very complex structures, but others are very simple -- like methane, CH4, which is the simplest possible (stable) compound of hydrogen and carbon, and which forms as more complex organic molecules break down.
In contrast, the material on Titan is believed to consist largely of methane, together with lesser amounts of moderately complex molecules, the amount decreasing with increasing complexity. This is the pattern one would expect if the compounds were originally derived from methane. In fact, since Titan lacks an ozone layer, there is nothing to shield its atmosphere from ultraviolet radiation, which is known to cause a number of chemical reactions initiated by the UV-induced cleavage of chemical bonds. In the case of CH4, H atoms are cleaved off, and recombine (more or less) at random, with molecular hydrogen (H2) being one of the likely products. Because H2 is so light, it can escape from Titan's gravity, leaving the heavier carbon-containing products behind. For example,
CH4 --> CH3 + H
2H --> H2 (escapes)
2CH3 --> C2H6
so the higher boiling, less volatile gas, ethane, is left behind. As the concentration of ethane increases, it is subjected to the same process, forming C3H8, C4H10, etc. Also, some products of other H-atom induced reactions will form, such as C3H6, C4H8, etc. The longer the UV (sunlight) works on the atmosphere, the more these intermediates build up, and the more complex they become, forming liquids and solids as well as gases. Leave Titan out in the Sun long enough, and most of its hydrogen will be lost, leaving the carbon-rich material behind. If Titan were closer to the Sun, the increased heat would help the lighter gases escape faster (hot gas molecules move faster) and so a hydrogen-depleted Titan would form sooner.
What would that look like? Probably Mars. Mars is closer to the Sun, and has too little gravity to hold onto the lighter gases. All the light gases -- H2, NH3 (ammonia), CH4, and H2O have either boiled off, or been photolyzed by UV. In the case of H2O, we know that even in Earth's outer atmosphere, UV splits of H atoms, which slowly diffuse away, leaving the oxygen behind (abiotic formation of O2!). On Mars, the gravity is too weak to hang onto even O2 long-term, so almost the only gas remaining is CO2, which is heavier than the other common gases.
Large planets -- the "gas giants" Jupiter, Saturn, Uranus, Neptune -- have atmospheres very rich in hydrogen, so much so that the other common elements are found predominately as binary compound with hydrogen. Thus the nitrogen on Jovians is in the form of NH3, the carbon in the form of CH4, and the oxygen in the form of H2O. To form more complex molecules, some of those hydrogen atoms have to be kicked out. On Jovians this process takes place to a small extent (leading to all those different colors in the atmosphere) but there is so much H2 present that the reverse process occurs at a competitive rate. On lighter bodies, the H2 can escape, leaving C,N,O, etc. to combine with each other. On Venus and Mars, that's left lots of CO2 (Venus is hot, Mars is small, so only the heaviest gas remains); on Earth, N2 and O2, and H2O (just big enough and cool enough for water), and on Titan, so far from the Sun, the loss of H2 will presumably take biiiiillions and biiiiillions of years more.
BTW, Carl Sagan and colleagues did some experiments in which they irradiated the mixture of gases postulated to be present on Titan with UV light for extended periods, and formed a dark reddish, tarry mixture which he named 'Titan tholin' (from the Greek work for mud).
More than you wanted to know, but what you wanted to know is a subset thereof.
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