There are multiple alternatives you available to you. The first is a stand alone app, the second is also stand alone but can integrate with Libre Office, and the third is built to integrate with Libre Office.
The first, available in the Ubuntu Software Centre, is a neat app called Referencer. With it you can organise documents or references, and ultimately generate a BibTeX bibliography file. Referencer includes a number of features to make this process easier.
The second, with many positive reviews in the USC, is JabRef. You can customize the entry information shown in the main window, and sort by any of the standard BibTeX fields. JabRef also lets you easily link to PDF or web sources for your reference entries. I great feature is that JabRef can integrate with Libre Office.
And third, Bibus, again in the Ubuntu Software Centre. Bibus is a bibliographic database which has been developed with LibreOffice in mind. It can directly insert citations and format the bibliographic index in an open Writer document.
Each one of these Apps excels in different ways, so try them all out. They available on Windows as well, if you ever have to do work on Windows and want the same tools.